Three cases of metastatic malignant lesions are presented. By conventional pathologic examination these lesions were diagnosed as undifferentiated carcinomas. They were studied using a chimpanzee antiserum specific for human melanoma-associated antigens. In each case, cells from the tumor were reactive with the anti-melanoma antiserum. The tumors were also studied using the DOPA test. All three tumors were positive in the DOPA test, suggesting the presence of tyrosinase within the tumor cells. The results suggest that serologic identification of melanoma may provide a rapid, inexpensive means for tumor diagnosis.