ABSTRACTThe roles of tyrosinase andγ‐glutamyl transpeptidase (γ‐GTP) in 5‐S‐cysteinyldopa (5‐S‐CD) genesis in melanotic and amelanotic melanoma cells has been investigatedin vitroandin vivo. Complete inhibition of tyrosinase by 10–3M diethyl dithiocarbamate has been found to partially inhibit dopa‐cysteine dependent 5‐S‐CD genesis by cultured melanoma cells. On the other hand iodoacetamide, an inhibitor ofγ‐GTP, has been found to completely inhibit dopa‐glutathione dependent 5‐S‐CD genesis by these cells. Melanotic melanoma has been found to be rich in tyrosinase andγ‐GTP activity. In contrast, the amelanotic melanoma studied here was found to lack tyrosinase activity and to have very littleγ‐GTP activity. The sub‐cellular distribution of these enzymes in melanotic melanoma cells has revealed that while the melanosome‐rich fraction is high in tyrosinase activity, it contains no substantialγ‐GTP activity. These findings indicate that 5‐S‐CD can form non‐enzymicallyin vitrofrom dopa and cysteine. Further, it also provides evidence thatγ‐GTP is the enzyme that conve