Human tyrosinase was partially purified from the lysate of a melanoma cell line and used to immunize BALB/c mice. Spleen cells from the immunized mice were fused with a murine myeloma cell line (NS-1), yielding a hybridoma (5C12) that produced monoclonal antibody directed against tyrosinase. 5C12 antibody reacted with normal human melanocytes, neval cells, primary cultures of melanoma biopsies, and most melanoma cell lines, including amelanotic lines with very low levels of enzyme activity. No reaction was found with keratinocytes, lymphoid cells, fibroblasts, and nonmelanoma cell lines. The 5C12 antibody was used to affinity-purify tyrosinase directly from a cell lysate, giving a single protein of 60,000 daltons, electrophoretically identical with enzyme activity and immunoreactivity with 5C12 antibody. Treatment of melanoma cells with periodate significantly reduced antigenicity. It can be inferred from these results that 5C12 antibody is directed against a carbohydrate moiety present in active and inactive tyrosinase, and that amelanotic melanoma cells may contain significant levels of the latter protein.