diff --git a/openapi_schema_pydantic/v3/v3_0_3/__init__.py b/openapi_schema_pydantic/v3/v3_0_3/__init__.py
index 666664a..c6013b4 100644
--- a/openapi_schema_pydantic/v3/v3_0_3/__init__.py
+++ b/openapi_schema_pydantic/v3/v3_0_3/__init__.py
@@ -17,7 +17,7 @@
 from .path_item import PathItem
 from .operation import Operation
 from .external_documentation import ExternalDocumentation
-from .parameter import Parameter
+from .parameter import Parameter, ParameterLocation
 from .request_body import RequestBody
 from .media_type import MediaType
 from .encoding import Encoding
diff --git a/openapi_schema_pydantic/v3/v3_0_3/header.py b/openapi_schema_pydantic/v3/v3_0_3/header.py
index c413cdc..6515f45 100644
--- a/openapi_schema_pydantic/v3/v3_0_3/header.py
+++ b/openapi_schema_pydantic/v3/v3_0_3/header.py
@@ -1,6 +1,6 @@
 from pydantic import Extra, Field
 
-from .parameter import Parameter
+from .parameter import Parameter, ParameterLocation
 
 
 class Header(Parameter):
@@ -14,7 +14,7 @@ class Header(Parameter):
     """
 
     name = Field(default="", const=True)
-    param_in = Field(default="header", const=True, alias="in")
+    param_in = Field(default=ParameterLocation.HEADER, const=True, alias="in")
 
     class Config:
         extra = Extra.ignore
diff --git a/openapi_schema_pydantic/v3/v3_0_3/parameter.py b/openapi_schema_pydantic/v3/v3_0_3/parameter.py
index 0f72df3..f0f8a37 100644
--- a/openapi_schema_pydantic/v3/v3_0_3/parameter.py
+++ b/openapi_schema_pydantic/v3/v3_0_3/parameter.py
@@ -1,3 +1,4 @@
+import enum
 from typing import Any, Dict, Optional, Union
 
 from pydantic import BaseModel, Field, Extra
@@ -8,6 +9,15 @@
 from .schema import Schema
 
 
+class ParameterLocation(str, enum.Enum):
+    """The location of a given parameter."""
+
+    QUERY = "query"
+    HEADER = "header"
+    PATH = "path"
+    COOKIE = "cookie"
+
+
 class Parameter(BaseModel):
     """
     Describes a single operation parameter.
@@ -30,7 +40,7 @@ class Parameter(BaseModel):
     - For all other cases, the `name` corresponds to the parameter name used by the [`in`](#parameterIn) property.
     """
 
-    param_in: str = Field(alias="in")
+    param_in: ParameterLocation = Field(alias="in")
     """
     **REQUIRED**. The location of the parameter. Possible values are `"query"`, `"header"`, `"path"` or `"cookie"`.
     """
diff --git a/openapi_schema_pydantic/v3/v3_1_0/__init__.py b/openapi_schema_pydantic/v3/v3_1_0/__init__.py
index dd938d1..702668d 100644
--- a/openapi_schema_pydantic/v3/v3_1_0/__init__.py
+++ b/openapi_schema_pydantic/v3/v3_1_0/__init__.py
@@ -17,7 +17,7 @@
 from .path_item import PathItem
 from .operation import Operation
 from .external_documentation import ExternalDocumentation
-from .parameter import Parameter
+from .parameter import Parameter, ParameterLocation
 from .request_body import RequestBody
 from .media_type import MediaType
 from .encoding import Encoding
diff --git a/openapi_schema_pydantic/v3/v3_1_0/header.py b/openapi_schema_pydantic/v3/v3_1_0/header.py
index c413cdc..6515f45 100644
--- a/openapi_schema_pydantic/v3/v3_1_0/header.py
+++ b/openapi_schema_pydantic/v3/v3_1_0/header.py
@@ -1,6 +1,6 @@
 from pydantic import Extra, Field
 
-from .parameter import Parameter
+from .parameter import Parameter, ParameterLocation
 
 
 class Header(Parameter):
@@ -14,7 +14,7 @@ class Header(Parameter):
     """
 
     name = Field(default="", const=True)
-    param_in = Field(default="header", const=True, alias="in")
+    param_in = Field(default=ParameterLocation.HEADER, const=True, alias="in")
 
     class Config:
         extra = Extra.ignore
diff --git a/openapi_schema_pydantic/v3/v3_1_0/parameter.py b/openapi_schema_pydantic/v3/v3_1_0/parameter.py
index e4f69c4..44efc9b 100644
--- a/openapi_schema_pydantic/v3/v3_1_0/parameter.py
+++ b/openapi_schema_pydantic/v3/v3_1_0/parameter.py
@@ -1,4 +1,5 @@
 from typing import Any, Dict, Optional, Union
+import enum
 
 from pydantic import BaseModel, Field, Extra
 
@@ -8,6 +9,15 @@
 from .schema import Schema
 
 
+class ParameterLocation(str, enum.Enum):
+    """The location of a given parameter."""
+
+    QUERY = "query"
+    HEADER = "header"
+    PATH = "path"
+    COOKIE = "cookie"
+
+
 class Parameter(BaseModel):
     """
     Describes a single operation parameter.
@@ -30,7 +40,7 @@ class Parameter(BaseModel):
     - For all other cases, the `name` corresponds to the parameter name used by the [`in`](#parameterIn) property.
     """
 
-    param_in: str = Field(alias="in")
+    param_in: ParameterLocation = Field(alias="in")
     """
     **REQUIRED**. The location of the parameter. Possible values are `"query"`, `"header"`, `"path"` or `"cookie"`.
     """
diff --git a/tests/test_alias.py b/tests/test_alias.py
index 2cd0b0f..256f2f3 100644
--- a/tests/test_alias.py
+++ b/tests/test_alias.py
@@ -1,11 +1,21 @@
-from openapi_schema_pydantic import Header, MediaType, Parameter, PathItem, Reference, Schema, SecurityScheme
+from openapi_schema_pydantic import (
+    Header,
+    MediaType,
+    Parameter,
+    ParameterLocation,
+    PathItem,
+    Reference,
+    Schema,
+    SecurityScheme,
+)
 
 
 def test_header_alias():
     header_1 = Header(param_in="header")
     header_2 = Header.parse_obj({"param_in": "header"})
     header_3 = Header.parse_obj({"in": "header"})
-    assert header_1 == header_2 == header_3
+    header_4 = Header.parse_obj({"in": ParameterLocation.HEADER})
+    assert header_1 == header_2 == header_3 == header_4
 
 
 def test_media_type_alias():