The reactivity of the monoclonal antibody HMB 45 was evaluated in melanocytic tumors of the conjunctiva. Among these are 10 acquired melanoses, 19 nevi and 34 melanomas. Results were compared with the presence of the S 100 antigen. Especially the intraepithelial and junctional components of primarily benign lesions were stained with HMB 45. Within malignant melanoma this antibody reacts with melanocytes in the epithelial, junctional and subepithelial areas. The polyclonal antibody S 100 stains all melanocytes in pigmented lesions of the conjunctiva. Intraepithelial or subepithelial malignant infiltrating tumor cells show very intense staining with HMB 45. HMB 45 has therefore high specificity for stimulated melanocytes, but it does not distinguish benign and malignant proliferating melanocytic cells.