Rendering a plane object as transparent

Hi,

I'm animating these objects:


And outputting a Quicktime movie with alpha but all I want it the objects and the shadows and for the plane (floor) to have alpha/transparency but I can't figure out how to do it.

I tried applying a compositing tag to the floor and deselecting 'seen by camera' but this didn't work as it would output the shadows.

Can you help?

Thanks.

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