Tyrosinase activity in the Ab hamster amelanotic melanoma cells cultured in serum-free Eagle's MEM increased 3 times after 6 h of primary cell culture. This increase was inhibited completely by cycloheximide, while actinomycin D had no effect on this process. After 24 h of culture in MEM with calf serum, further increase of the tyrosinase activity was inhibited by both cycloheximide and actinomycin D. The data presented may indicate that the increase of tyrosinase activity in the primary cell culture of the Ab melanoma is due initially to the unblocking of translation and later to the activation of transcription of the gene controlling the enzyme.