The H2 breath test was performed by ingestion of 33 g lactulose and analysis of end-expiratory air. Eight of 100 healthy subjects failed to produce significant amounts of H2. The test was repeated twice in these eight subjects. Four had a flat excretion curve in all three tests, and all excreted large amounts of breath CH4. Of the 100, 34 had breath CH4 above 0.5 mumol/l. They had significantly lower fasting breath H2 and breath H2 excretion after lactulose than the 66 with breath CH4 below 0.5 mumol/l (p less than 0.01). By means of a gas chromatographic method with high sensitivity for CH4, 33 of the 100 subjects were restudied with parallel measurement of pulmonary excretion of H2 and CH4. Typical patterns of excretion were found in the subjects with endogenous CH4 production, showing either high excretion of H2 and low CH4 or low H2 and high CH4. A combination of high H2 and high CH4 was never seen. These findings suggest that CH4 is produced in the human intestine chiefly by an H2-utilizing flora and that adequate assessment of gut bacterial carbohydrate fermentation would require parallel measurement of breath H2 and CH4. The prevalence of CH4 production in a group of 120 healthy subjects, determined by a single midday breath sample, was 44%, with no significant difference between sexes and no correlation to age. Repeated midday breath sampling in 12 subjects during 1 month proved this method to be very reliable in the assessment of an individual's ability to produce CH4.