pixmap transparency png

[Fawcett, David]

I am trying to use a png image as a pixmap symbol with transparency. 

MapServer 4.6.1 

The symbol renders, but I can't get the background to go transparent.  I
looked up the color index in photoshop and it looks like the background
color is number 255.  

My layer looks like:

LAYER
    NAME spots
    STATUS DEFAULT
    TYPE POINT
    DATA 'spotfile'
    CLASS 
       NAME "None"
	   STYLE
	      Symbol "greendot"
	   END
    END
END

The symbol file looks like:  

SYMBOL
  NAME "greendot"
  TYPE PIXMAP
  IMAGE "DotGreen.png"
  TRANSPARENT 255
END

Any ideas what could be going wrong?  

David.