The aim of this work was to determine the effect of yeast extract and of its vitamin contents on autotrophic and heterotrophic growth and metabolism of four acetogenic bacteria from the human colon. Yeast extract exerted a stimulatory effect on autotrophic growth of the colonic acetogens, but concentration of this compound above 1-2 g. L-1 in the medium did not enhance utilization of H-2/CO2 Vitamins provided by yeast extract were shown to be essential cofactors of the reductive pathway of acetate synthesis except for one Clostridium strain. Yeast extract was also necessary to maintain heterotrophic growth and acetate synthesis from glucose in acetogenic species, except in the Streptococcus strain. In the absence of yeast extract, vitamins could efficiently restore glucose fermentation via acetate. The reductive and oxidative pathways of acetate synthesis might, therefore, depend on vitamin cofactors supplied by yeast extract in most of the human acetogenic bacteria. Non-vitaminic factors appeared also to be involved in the metabolism of some of these acetogenic species.