External movie

I have a movie called ‘B’. Inside ‘B’ is a movie clip called ‘b1’. ‘B’ is loaded onto level 2 of main movie called ‘A’. Inside ‘A’ is a clip called ‘a1’. When ‘b1’ is finished playing I want ‘a1’ to start playing, how do I do this. Would I use _parent or what?
Thanx

At the last frame of b1, place
[AS]
_root.a1.gotoAndPlay(1);
[/AS]

That would do. I suppose A & B are names you give to the files, they are not instances of movieclips.

A & B are just names I use for explaining purposes. The real names are a bit long.