The effects of exogenous thiamin on the growth yield and vitamin B6 content of 18 strains of yeasts and a few strains of bacteria were examined. The addition of thiamin hardly affected the growth yield of the yeasts tested, except for two strains Saccharomyces uvarum strain 4228 and Saccharomyces uvarum IFO 0751. In contrast, the vitamin B6 content of all the yeasts tested, except Pichia membranaefaciens IFO 0189, decreased markedly in the presence of thiamin. In S. uvarum IFO 1265, the synthesis of vitamin B6 was maximally inhibited by the addition of thiamin (1.5 nmol ml-1) to the growth medium without affecting cell growth, whereas the amounts of cellular vitamin B6 increased in the presence of the thiamin antagonist, pyrithiamin or oxythiamin, at concentrations that did not affect growth. When [4'-14C]pyridoxine. HCl (0.5 micrograms ml-1) was added to the growth medium at least 54% of the added isotope was incorporated into the cells during 24 h incubation. In the presence of thiamin (15 nmol ml-1), at least 32% of the added isotope was incorporated. The metabolism of [4'-14C]pyridoxine. HCl to inactive forms having no vitamin B6 activity was not stimulated by the addition of thiamin. Thus, vitamin B6 synthesis in many yeasts was affected by thiamin, whereas, in bacteria, growth yield and vitamin B6 content were not affected by thiamin.