As described previously (H. Togashi et al. Biochem Pharmacol. 1998;56:583–590), the irradiated products of provitamin D2 (ergosterol) inhibit the activities of eukaryotic DNA polymerases. In this report, therefore, we investigated whether vitamin D and its related compounds inhibited the activities of DNA polymerases. As expected, vitamin D2 and vitamin D3 were found to be selective inhibitors of mammalian DNA polymerase a (pol a) with IC50 values of 123 and 96 μM, respectively. On the other hand, provitamin D2, provitamin D3, and the active form of vitamin D3 such as 1a,25-dihydroxyvitamin D3 could not influence any of the DNA polymerase activities. Interestingly, vitamin D3-3β-sulfate was a much stronger pol a inhibitor with an IC50 value of 7.1 μM. Vitamin D2, vitamin D3, and vitamin... D3-3β-sulfate could prevent the growth of NUGC-3 human gastric cancer cells with LD50 values of 133, 77, and 44 μM, respectively, but provitamin D2 and provitamin D3 could not. The cells were halted at the G1 phase in the cell cycle by these compounds.