Abstract: This invention concerns novel receptor protein tyrosine phosphatase polypeptides. Specifically, this invention concerns the novel receptor protein tyrosine phosphatase .lambda. which is related to the homotypically adhering receptor protein tyrosine phosphatases .kappa. and .mu.. The invention further relates to analogs of these polypeptides in other mammals, functional derivatives thereof, antibodies which are capable of specifically binding to these polypeptides, nucleic acids encoding these polypeptides, vectors containing and capable of expressing such nucleic acid and recombinant host cells transformed with such nucleic acid. Methods for the recombinant production of these receptor protein tyrosine phosphatase polypeptides and assays for identifying agonists and antagonists of these polypeptides are also within the scope of the invention.