The pigmented human melanoma cell line, MM418, became demelanized when treated continuously with a nontoxic level of halistanol trisulphate (HTS), a C29 steroidal detergent isolated from a marine sponge. Nontoxic levels of halistanol or of a range of anionic, cationic and neutral detergents had no such effect. Control MM418 cells varied greatly in size, appearance and pigmentation; HTS-treated cells were smaller than controls, had a uniform, generally bipolar appearance, and lacked pigment. HTS induced only minor changes in cell ultrastructure, with fewer mature melanosomes being found in treated cells. Suppression of melanin synthesis was apparent within 24 h of addition of HTS, as judged by inhibited incorporation of the false precursor, 5[125I]-2-thiouracil. Reversal of inhibition occurred within the same period after removal of HTS. Tyrosinase activity gradually decreased to 25% of the control value during a 19-day treatment with HTS, and expression of two-carbohydrate-dependent tyrosinase epitopes, 5C12 and 2B7, was abolished. Expression of one other melanosomal protein and of vimentin was not affected. The results suggest that HTS inhibits maturation of tyrosinase to a form associated with melanin synthesis.