A monoclonal antibody (MAb NKI/C-3) produced against a purified membrane preparation of human melanoma cells reacts preferentially with sections of formaldehyde-fixed and paraffin-embedded tissues of melanoma, nevocellular nevi, carcinoids and medullary carcinomas of the thyroid. NKI/C-3 did not react with basal-cell carcinoma, brain tissue or brain tumors, and in only 14/196 other tumors was a clear cross-reactivity observed, e.g. with prostate carcinomas and a minority of primary breast, ovarian, lung and clear-cell carcinomas. This antibody was used in an immuno-electron microscopic study for the cellular localization of the antigen. The antigen was dispersed in the cytoplasm of melanoma cells, and more concentrated inside vacuoles and sometimes also on the melanosomes. Occasionally, the antigen was seen on the cell surface. The nature of the antigen was determined in an enzyme immunoassay (EIA). It was found that the antigen is a glycoprotein with a disulfide-dependent configuration that is essential for recognition by the MAb. The antigen was distributed heterogeneously during gel filtration as well as during SDS-polyacrylamide gel electrophoresis in the region of 25-110 kd proteins. A purified antigen preparation that was obtained after affinity chromatography on a column of MAb NKI/C-3 linked to Sepharose 4B contained a carbohydrate:protein ratio of 1:3.5.