Antisera to common human melanoma antigens were obtained from melanoma patients receiving autologous immunization with their own irradiated cultured melanoma cells and Bacillus Calmette-Guérin. The antibody thus derived was used to detect common antigens on the plasma membrane of three different human melanoma cell lines by membrane immunofluorescence. The antigen-antibody complexes on the surface of melanoma cells would move to a pole (capping) and would subsequently be extruded into the extracellular milieu at room temperature. Approximately 25 to 30% of viable cells were positive by immunofluorescence. However, when the cells were fixed with methanol, 60 to 70% of cells demonstrated membrane binding. Capping was inhibited at 0 degrees or when the cells were pretreated with vinblastine sulfate. It can be concluded that common tumor antigens exist on the surface of viable human melanoma cells and that the redistribution of antigen-antibody complexes is an active process. The extrusion of antigen-antibody complexes in vitro may represent a mechanism of antigenic modulation in vivo and could indicate a basic method of tumor survival since presumably the antigen-denuded cell is viable and capable of replication but not of recognition by subsequent effector immune events.