The 89-kDa cell surface glycoprotein, P3.58, is detectable on advanced human melanomas in situ but not on benign melanocytes or early melanomas. cDNA cloning of P3.58 from melanoma cells was accomplished by screening a lambda zap expression vector library with monoclonal antibodies produced against the denatured antigen. Nucleotide sequencing of the clones revealed that P3.58 is identical to the intercellular-adhesion molecule 1. No qualitative differences in P3.58 mRNA species could be seen between melanoma cells and hematopoietic cells and no differences in gene organization were observed between peripheral blood leukocytes and melanoma cells. Inspection of the deduced amino acid sequence of P3.58 indicated the presence of the consensus sequence characteristic for complement-binding proteins. The acquisition of this cell-adhesion molecule during the process of tumor progression is speculated to contribute to the development of metastasis in melanoma.