A platin medium containing cephalothin and clindamycin was developed for enumeration and isolation of methanogens in human feces. Specimens from nine CH4-producing subjects had total anaerobe counts of 1–8×1011/g dry weight. Methanogen counts on the antibiotic medium ranged from 0.001–12.6% of the total anaerobe count. There was no correlation between age, sex, or percent dry fecal weight and the ratio of methanogens to total counts. Specimens from eight non-CH4-producing individuals contained bacteria thay yielded nonmethanogenic colonies on the antibiotic medium. The means±SD of the logarithm of the total counts per gram dry weight were 11.4±0.29 and 11.38±0.44 for the positive and negative groups respectively. Values for the antibiotic-resistant flora were 8.8±1.13 and 7.78±1.08 respectively. Methanogens were isolated from the most dilute inoculum of each specimen from CH4-producing subjects. All isolates were morphologically, physiologically, and immunologically identical to Methanobrevibacter smithii. Growth of methanogens in media that were essentially extracts of CH4-negative feces suggested that no nutrients were lacking or inhibitors present in intestinal contents that prevent the growth of methanogens in these individuals.