combining layers with erased backgrounds

When I save an image with a part of it having been erased the image saves with white replacing the erased portion.  I have two such images, as layers, which I want to merge but I cannot get either image back to the image wthout the background.  I cannot get rid of the "white" part that is now what the erased part was.  I have done this in the past but dont not remember how!  Thanks for any help.