The metabolism of carbon monoxide by the colonic flora was investigated using human fecal homogenates. During anaerobic incubation, these homogenates rapidly consumed added carbon monoxide reducing the PCO level to a minimum of about 0.2 ppm. In the presence of glucose, carbon monoxide consumption averaged about 0.7 ml/h · g feces and without glucose about 0.2 ml/h · g feces. This consumption was not observed if the homogenates were autoclaved, passed through a bacterial filter, or cultured aerobically, indicating that the carbon monoxide was removed by the metabolism of fecal anaerobes. Aerobic incubation of fecal homogenates resulted in slow but definite release of carbon monoxide. While bacterial carbon monoxide consumption probably does not play an appreciable role in the turnover of carbon monoxide that is inhaled or exogenously produced, it is possible that carbon monoxide uptake by colonic flora protects other fecal organisms and possibly the host from carbon monoxide liberated in the gut.