Membrane antigens of a cultured human melanoma line, UCLASO-M14, were studied using immune adherence techniques. Allogeneic sera from melanoma patients that were reactive with the M14 but nonreactive with lymphoid cells of the M14 donor were used as antibodies. The antigen responsible for the reaction between M14 and the antibodies was searched for in other cancer, normal, and fetal tissues using antibody absorption techniques. The antigen was found in a variety of different histological types of biopsied and cultured cancer cells as well as in melanomas. The antigen did not exist in biopsied normal tissues, but it appeared in cultured normal skin and muscle. Neither normal lymphocytes nor cultured lymphoid cells showed any antigenicity. The antigen was present in human fetal tissues and was the strongest in fetal brain tissues at 22 weeks of development. Liver, spleen, thymus, and small intestine from the same fetus were negative for antigen.