A mouse monoclonal antibody, FKH1, was produced to detect cytoplasmic melanoma-associated antigen. FKH1 was raised using cultured human melanoma cell line KHm-6 as an immunogen. Reactivity of this antibody was assessed by immunohistochemical techniques against cell lines and normal and neoplastic tissues. Positive reactions were seen against 5 human melanoma cell lines. It stained cytoplasm of melanoma cells in a diffuse and granular pattern in indirect immunofluorescence. Immunoelectron microscopy showed diffuse distribution of immunoreactant in the cytoplasm of KHm-1 cells excluding melanosomes and other organelles. Reactivity against frozen and alcohol-fixed, paraffin-embedded melanocytic tumors was also tested with IIF or indirect or avidin biotinylated horseradish peroxidase complex immunoperoxidase techniques. All cases of frozen sections from benign and malignant melanocytic tumors showed positive staining with FKH1. In fixed tissues, however, reactivity was 11 of 14 (79%) in malignant melanoma and 28 of 42 (67%) in other melanocytic tumors. FKH1 did not react against normal melanocytes and nonmelanocytic tumors except APUDoma and 2 glioblastoma cell lines. It failed to stain the B-16 mouse melanoma cell line, neuroblastoma cell line, breast carcinoma cell line, and T-cell lymphoma cell line. Normal human peripheral nerves were nonreactive with FKH1. In immunoelectroblot study, FKH1 bound with proteins having molecular weight of 71,000 and 55,000 extracted from KHm-6 cells. It was suggested that FKH1 is a useful monoclonal antibody in diagnostic study of human malignant melanoma specimens.