diff --git a/src/constants.cpp b/src/constants.cpp
index fdfaa847..66ed95f3 100644
--- a/src/constants.cpp
+++ b/src/constants.cpp
@@ -172,6 +172,7 @@ namespace fields
 	const string::value_type* const CONTENT_DISPOSITION = "Content-Disposition";
 	const string::value_type* const CONTENT_ID = "Content-Id";
 	const string::value_type* const CONTENT_LOCATION = "Content-Location";
+	const string::value_type* const IN_REPLY_TO = "In-Reply-To";
 
 	const string::value_type* const X_MAILER = "X-Mailer";
 	const string::value_type* const X_PRIORITY = "X-Priority";
diff --git a/vmime/constants.hpp b/vmime/constants.hpp
index 0451408f..607ad2c8 100644
--- a/vmime/constants.hpp
+++ b/vmime/constants.hpp
@@ -175,6 +175,7 @@ namespace vmime
 		extern const string::value_type* const CONTENT_DISPOSITION;
 		extern const string::value_type* const CONTENT_ID;
 		extern const string::value_type* const CONTENT_LOCATION;
+		extern const string::value_type* const IN_REPLY_TO;
 
 		extern const string::value_type* const X_MAILER;
 		extern const string::value_type* const X_PRIORITY;
diff --git a/vmime/header.hpp b/vmime/header.hpp
index 1895e79e..3f80c96b 100644
--- a/vmime/header.hpp
+++ b/vmime/header.hpp
@@ -67,6 +67,7 @@ public:
 	FIELD_ACCESS(Sender,                  SENDER,                    mailboxField)
 	FIELD_ACCESS(ReplyTo,                 REPLY_TO,                  mailboxField)
 	FIELD_ACCESS(DeliveredTo,             DELIVERED_TO,              mailboxField)
+	FIELD_ACCESS(InReplyTo,               IN_REPLY_TO,               messageIdField)
 
 	FIELD_ACCESS(To,                      TO,                        addressListField)
 	FIELD_ACCESS(Cc,                      CC,                        addressListField)