diff --git a/PIL/FpxImagePlugin.py b/PIL/FpxImagePlugin.py
index d369e05fa..aefc57420 100644
--- a/PIL/FpxImagePlugin.py
+++ b/PIL/FpxImagePlugin.py
@@ -221,6 +221,6 @@ class FpxImageFile(ImageFile.ImageFile):
 #
 # --------------------------------------------------------------------
 
-Image.register_open("FPX", FpxImageFile, _accept)
+Image.register_open(FpxImageFile.format, FpxImageFile, _accept)
 
-Image.register_extension("FPX", ".fpx")
+Image.register_extension(FpxImageFile.format, ".fpx")
diff --git a/PIL/ImtImagePlugin.py b/PIL/ImtImagePlugin.py
index 1ca2c25b0..63e892483 100644
--- a/PIL/ImtImagePlugin.py
+++ b/PIL/ImtImagePlugin.py
@@ -89,7 +89,7 @@ class ImtImageFile(ImageFile.ImageFile):
 #
 # --------------------------------------------------------------------
 
-Image.register_open("IMT", ImtImageFile)
+Image.register_open(ImtImageFile.format, ImtImageFile)
 
 #
 # no extension registered (".im" is simply too common)
diff --git a/PIL/McIdasImagePlugin.py b/PIL/McIdasImagePlugin.py
index 705fa574f..b75360353 100644
--- a/PIL/McIdasImagePlugin.py
+++ b/PIL/McIdasImagePlugin.py
@@ -69,6 +69,6 @@ class McIdasImageFile(ImageFile.ImageFile):
 # --------------------------------------------------------------------
 # registry
 
-Image.register_open("MCIDAS", McIdasImageFile, _accept)
+Image.register_open(McIdasImageFile.format, McIdasImageFile, _accept)
 
 # no default extension
diff --git a/PIL/PixarImagePlugin.py b/PIL/PixarImagePlugin.py
index 7fef35408..db2ee55f8 100644
--- a/PIL/PixarImagePlugin.py
+++ b/PIL/PixarImagePlugin.py
@@ -62,7 +62,7 @@ class PixarImageFile(ImageFile.ImageFile):
 #
 # --------------------------------------------------------------------
 
-Image.register_open("PIXAR", PixarImageFile)
+Image.register_open(PixarImageFile.format, PixarImageFile)
 
 #
 # FIXME: what's the standard extension?
diff --git a/PIL/XVThumbImagePlugin.py b/PIL/XVThumbImagePlugin.py
index 9d4b704f6..311e65dc0 100644
--- a/PIL/XVThumbImagePlugin.py
+++ b/PIL/XVThumbImagePlugin.py
@@ -72,4 +72,4 @@ class XVThumbImageFile(ImageFile.ImageFile):
 
 # --------------------------------------------------------------------
 
-Image.register_open("XVThumb", XVThumbImageFile)
+Image.register_open(XVThumbImageFile.format, XVThumbImageFile)