The antigenic heterogeneity of primary and metastatic lesions surgically removed from nine patients with nodular melanoma was investigated by using monoclonal antibodies to HLA-A, B antigens, to beta 2-microglobulin, to Ia antigens, and to melanoma-associated antigens (MAA). The latter include three types of membrane-bound MAA and a cytoplasmic MAA. In spite of an homogeneous morphologic appearance, multiple lesions removed from the same patient differed significantly in their reactivity with the panel of monoclonal antibodies in indirect immunofluorescence test. The extent of antigenic heterogeneity did not correlate with melanin synthesis, site of origin of the primary tumor, site of metastatic foci, or treatment, but was less marked in patients carrying the primary tumor. The antigenic heterogeneity of multiple lesions removed from one patient and the independent expression of the various types of MAA investigated suggest that combinations of monoclonal antibodies to MAA may be more effective than single antibodies for radioimaging and immunotherapy.