A convenient, rapid, and reproducible assay was developed to evaluate the immunoreactivity of radiolabeled monoclonal antibodies against three different human melanoma-associated antigens, p97, a proteoglycan and a GD3 ganglioside. A cloned melanoma cell line (M 2669 CL 13) was selected as the target and, when fixed with paraformaldehyde, showed binding as good as or better than that obtained with live cells for the three antigens. Fixed cells retained good binding properties stored at 4 degrees C for over 6 mo. This assay has general applicability to other antigen-antibody systems for testing chemically modified monoclonal antibodies or fragments during the development of a radiopharmaceutical or as a routine quality control measure for clinical agents.