This study reports our success in developing a murine monoclonal antibody (mAb) against phaeomelanin and its major precursor, 5-S-cysteinyldopa (5-S-CD). A competitive enzyme-linked Immunosorbent assay was developed and demonstrated that the new mAb, designated as 6D4 (lgG1, k), reacted with both 5-S-CD and phaeomelanin, but not with eumelanin. The concentrations required for 50% inhibition of 5-S-CD and phaeomelanin were 65 ng/ml and 95 ng/ml respectively. The minimal amount of 5-S-CD and phaeomelanin which could be detected by mAb 6D4 was approximately 5 and 6 ng/ml, respectively. The immunohistochemical assay indicated that the antigenic epitope(s) recognized by mAb 6D4 was localized In the cytoplasm of certain types of melanocytlc tumours, such as superficial spreading melanoma.