Fresh feces from 27 healthy infants-12 breastfed (complete, exclusive breast-feeding), 7 in early weaning (partial, high breast-feeding), and 8 in late weaning (partial, low breast-feeding)-were cultured with simple and complex carbohydrates in vitro to test the hypothesis that colonic fermentation capacity for carbohydrates increases during weaning. Infants in all three groups were able to ferment sugars, with no significant differences in median total short-chain fatty acid (SCFA) concentrations (mmol/L): preweaning, 56.4(range: 0-77.6); early weaning 68.5(range: 57.9-98.8); late weaning, 61.3(range: 28.6-120.4) for glucose. Preweaned infants were less able to ferment oligosaccharides and complex carbohydrates than were weaned infants (P < 0.05). Ability to ferment raftilose was higher in early weaning; median total SCFA concentrations (mmol/L) were as follows: preweaning 31.0 (range: 3.6-48.9), early weaning 57.1 (range: 2.5-70.6), late weaning 68.6 (range: 22.0-113.4) (P < 0.05). Ability to ferment complex carbohydrates did not develop until late weaning; median total SCFA concentrations for guar gum (mmol/L) were as follows: preweaning 6.4 (range: 0.1-57.3), early weaning 18.4 (range: 0.0-40.5), late weaning 45.4 (range: 15.6-62.1) (P < 0.05, preweaning and early weaning compared with late weaning). Development of the ability to ferment complex carbohydrate was slow. Cultures of feces from preweaned infants produced eight times more SCFAs with glucose than with complex carbohydrates, at early weaning there was a threefold difference and by late weaning the difference was only 25%, but this was still only 42% of the SCFAs produced by cultures of adult feces. These data suggest that for the complex carbohydrates tested, colonic fermentation is likely to contribute only a small proportion of daily energy needs of weaning infants.