To identify molecules which may be functionally associated with the development of metastasis in human melanoma, monoclonal antibodies which discriminate between benign and malignant melanocytic lesions in situ were selected. Biochemical studies and cDNA cloning identified the antigens HLA-DR, ICAM-1 and MUC18 which showed an expression pattern on primary tumors correlating with vertical tumor thickness, the most predictive parameter for the development of metastasis in melanoma. ICAM-1 and MUC18 show sequence similarity to a family of cell adhesion molecules which include the neural cell adhesion molecule NCAM. Both HLA-DR and ICAM-1 can be induced on melanoma cells by lymphokines, suggesting a role of the mononuclear cell infiltrate in the control of tumor cell phenotype. Knowledge of the normal function of these molecules allows one to hypothesize how they may contribute to the successful development of metastases.