Tumours induced by physical or chemical carcinogens often express tumour-specific antigens that can induce strong protective immune defence in the host. The diversity of these unique antigens among different tumours is seemingly endless, and has been compared to that of immune receptors. At present, the nature and complexity of this antigenicity is not known for any single tumour. Here we describe the unique antigenicity expressed by a murine ultraviolet light (UV)-induced fibrosarcoma. This tumour is clearly subject to immune surveillance by the normal host, and does not grow progressively unless it undergoes antigenic changes. Using defined monoclonal T-cell probes and tumour variants selected in vitro with these probes, we found that the total antigenicity consisted of multiple independent components, all of which were tumour-specific and expressed simultaneously on the same tumour cell. The demonstration of this antigenic complexity will enable us to identify and compare the molecular composition of the components of this antigen, as well as to determine their individual roles in tumour rejection and escape.