Human fecal bacteria produce and consume hydrogen, and consumption rate is a critical determinant of the volume of H2 liberated into the colonic lumen. Two bacterial reactions that have been reported to be major consumers of H2 are methanogenesis and sulfate reduction. To determine the importance of each of these reactions, we measured the disappearance of exogenous H2 during incubation with human fecal homogenates treated with 20 mmol/L 2-bromoethanesulphonic acid (BES), an inhibitor of methanogenesis, and/or 20 mmol/L sodium molybdate (Mo), an inhibitor of sulfate reduction. Four methanogenic and four nonmethanogenic samples consumed an average of 99% and 85%, respectively, of the initial H2 during 22 hours of incubation. With methanogenic homogenates, 36% of the H2 consumption persisted despite inhibition of methanogenesis. Inhibition of sulfate reduction had no effect on the rate of H2 consumption by these eight fecal specimens. The importance of fecal sulfate availability was determined in fecal samples obtained from an additional 14 randomly selected volunteers. Incubation after supplementation with 20 mmol/L sulfate was associated with an increase in sulfide production in four of the samples, and three of these four samples also demonstrated an increased rate of H2 consumption. No such evidence of sulfate reduction was observed in the other 10 homogenates. We conclude that a bacterial reaction other than methanogenesis and sulfate reduction, perhaps the recently described reduction of CO2 to acetate, represents a major metabolic route of H2 disposal in nonmethanogenic feces and a minor, but appreciable, pathway in methanogenic feces.