Monoclonal antibodies that specifically bind to the tetrameric form of the alpha-folate receptor and not the monomeric form are provided. The antibodies are useful in the treatment of certain cancers, particularly cancers that have increased cell surface expression of the alphafolate receptor ('FR-a'), such as ovarian cancer. Hybridoma cells expressing the monoclonal antibodies, antibody derivatives, such as chimeric and humanized monoclonal antibodies, antibody fragments, mammalian cells expressing the monoclonal antibodies, derivatives and fragments, and methods of detecting and treating cancer using the antibodies, derivatives and fragments also are provided.