Employing as immunogen a short-term passaged, highly pigmented human melanoma cell line, we have produced the murine MoAb 2G10 of the IgG1 isotype. The antibody immunoprecipitated from 35S-methionine and 3H-glucosamine metabolically labeled human melanoma cells with a single-chain glycoprotein of 75 kD molecular weight. No such molecule could be precipitated from murine melanomas. To further investigate the fine specificity of the MoAb, immunochemical and immunohistochemical studies were performed. These studies demonstrated that MoAb 2G10 binds a significant fraction of tyrosinase activity from cell lysates, completely immunodepletes soluble cell extract of T4-tyrosinase molecules, and produces immunostaining patterns superimposable on those obtained with anti-T4-tyrosinase antibodies. Thus, MoAb 2G10 appears to recognize a human-specific determinant carried by either T4-tyrosinase or a closely related molecule. The functional relevance of this epitope remains to be evaluated.