The fermentation to short-chain fatty acids, lactate, and ammonia from several non-starch polysaccharides, glucose, and albumin was investigated in 16.6% faecal homogenates. Increasing concentrations (0-30 mg/ml) of glucose, wheat bran, pectin, ispaghula, cellulose, or albumin incubated for 24 h in homogenates pooled from three individuals increased short-chain fatty acid production linearly. Amounts and ratios of short-chain fatty acids formed were highly dependent on the type of substrate fermented. Fermentable saccharides increased ammonia assimilation, in contrast to the metabolic inert cellulose. Nine faecal homogenates sampled from three individuals at three occasions were incubated for 6 and 24 h. The production of total short-chain fatty acids, acetate, propionate, and butyrate and the accumulation of D- and L-lactate changed considerably in relation to the type of substrate added (cellulose, ispaghula, wheat bran, pectin, gum arabic, and glucose; p less than 10(-4)-10(-7). In contrast, there were no significant (p greater than 0.05) differences in organic acid formation between the nine homogenates, and the intra- and inter-individual variations were of the same magnitude. Variations in fermentation, when measured as organic acid formation, were therefore related to the type of substrate fermented rather than the individual tested.