Depends on the target application. As Mark mentioned Targas carry the alpha information over well. Not just in Carrara but also when I use them in Lightwave. One thing that would be nice is to have the option of seperate alphas. In Lightwave I can save individual RGB frames in a number of different formats, I also have the option of saving the alpha information as seperate frames. That way I dont have to worry about the target application, I just select the alpha's to use as a mask. The masks always come out clean when I do it this way. One drawback is that it creates more files though.