Hi, i Have an object on a white background. How can i replace the white for nothing. I will export this object in FCP ant key it. Thanks

Is the layer the object is on called "Background?" If so, rename it to something else. The "Background" layer cannot have empty (transparent) pixels. If you attempt to delete them, the pixels are painted with the background color.

go to the layers palette, dubbelclick on the background layer, the layer becomes layer 0, you can now select and delete or move partly or entirely your layer. your background layer became a normal layer.

Once you have a workable layer, it seems like the best option would be to Magic Wand the white area that you want to delete. Making a selection by Colour Range might give you more control. If you need to make the selection manually, try the Magnetic Lasso tool.