Significant levels of intracellular catecholamines were found in a human melanoma cell line and were enhanced by increasing the extracellular tyrosine concentration. Intracellular dopa, 5-cysteinyldopa, tyrosinase, and melanin also rose under these conditions. 5-HT (serotonin) was synthesized by the melanoma cells but further study was hindered by the high level of 5-HT in fetal calf serum. A 5-HT uptake antagonist, DU 24565 (6-nitroquipazine), was employed as an alternative method for studying 5-HT action. This compound, which in contrast to tunicamycin had no inhibitory effects on cell proliferation or tyrosinase activity, strongly inhibited melanization and decreased the levels of dopa, 5-cysteinyldopa, dopamine, noradrenaline, adrenaline, and 3,4-dihydroxyphenylacetic acid. DU 24565 had little effect on 5-HT or tyrosine accumulation in these cells but suppressed the uptake of extracellular dopa. The results show that human melanoma cells synthesize a wide range of biogenic amines in culture and suggest a new approach to regulating intracellular levels of dopa and of a variety of dopa products.