The growth rate of three vitamin B6-dependent Streptococcus mitior (B6DS) and two non-B6DS strains in Todd-Hewitt broth, with and without vitamin B6 supplementation, was examined. Even in optimally supplemented culture media, the growth rate of the three B6DS strains was much slower than that of comparable non-B6DS strains. Uptake studies with [3H] pyridoxine suggest that these B6DS strains cannot assimilate pyridoxine. Although not transported intracellularly, pyridoxine inhibited the growth of B6DS strains in media supplemented with other vitamin B6 analogs, probably by binding to the vitamin B6 transport system and inhibiting the uptake of the other vitamin B6 analogs.