AbstractThe effect of vitamin B6on the growth of B16 melanoma cells in vivo and in vitro was studied. B16 melanoma cells grown for three days in medium supplemented with 5.0 mM pyridoxine or 0.5 mM pyridoxal showed an 80% reduction in cell proliferation compared with control culture. Cells cultured for six hours in medium supplemented with 0.5 mM pyridoxal took up and incorporated 13 and 32% less [3H]thymidine, respectively, than did control cultures. A 17% reduction in [3H]glucose uptake was observed at this time point. When the incubation time was decreased to three hours, an inhibition of cellular uptake of [3H]thymidine (22%), [3H]uridine (14%), and [3H]glucose (15%) was observed; however, little or no inhibition in incorporation was detected. In in vivo studies, mice pretreated with pyridoxal for two weeks and then injected with B16 melanoma cells had a 62% reduction in tumor weight compared with controls at the end of a three‐week period. If tumors were first established in mice and then treated with pyridoxal for six days, a 39% reduction in tumor growth was observed. There were no differences observed in body weights or liver weights in any of the animal groups.These results indicate that supraphysiological doses of vitamin B6can inhibit the growth of B16 melanoma cells both in vitro and in vivo. The exact mechanism by which pyridoxal exerts its inhibitory effect was not ascertained, but experiments suggest that the vitamer may be acting on the plasma membrane to reduce precursor transport into the cell.