Transparency png does not work correct

Hello,

I'm using a png image with transparency as a surface and render it directly to a white background.
But there is no fading from 100% transparent to 0%. It is ever 0 or 255, no 120 for example.
I'm using svn #872 and OpenTK for rendering.

Edit:
My fault, sorry, forgot to clear the screen before each draw :S