Problem with _alpha

I have a really simple website I’m making. If you roll over a movieclip that has 5 buttons in it there’s a function that increases the alpha. For some reason though my alpha settings aren’t doing anything. Even if I set the alpha manually to zero and it disappears on the stage, when I run the movie it’s still showing. What’s going on?

hit_mc.onRollOver = function() {
this._alpha = 65;
};
hit_mc.onRollOut = function() {
};

This is it. I know it’s running the function and it even says the alpha is changed if I put a trace in.