12–0-tetradecanoylphorbol-13-acetate (TPA), a potent tumor promoter, stimulated membrane transport of choline in cultured cells. This is one of the earliest phenomena caused by tumor promoter. 4-O-methyl-TPA and mezerein showed similar stimulatory effects, but 4β-phorbol, which has no tumor-promoting activity, did not show any significant effect on choline transport. A calmodulin antagonist, N-(6-aminohexyl)-5-chloro-l-naphtalenesulfonamide, inhibited the TPA-stimulated transport of choline, whereas the protein kinase C inhibitor l-(5-isoquinolinesulfonyl)-2-methylpiperazine did not. These results suggest that the Ca2+-calmodulin system, but not protein kinase C, may play an important role in the mechanism of this phenomenon.