Digestive bacterial microflora play a major role in the pathogenesis of Crohn's disease (CD). Bacterial enzyme activities, especially beta-D-galactosidase, are decreased in fecal extracts from CD patients. We hypothesized that an alteration of the colonic flora might be responsible for this decrease. Indeed, we demonstrate that beta-D-galactosidase production in supernates of anaerobic cultures was significantly (P < 0.01) reduced in feces from patients with active Crohn's disease (N = 7), when compared to healthy controls (N = 8). Therefore using X-gal and selective media, we enumerated bacteria able to release beta-D-galactosidase in feces from patients with active (N = 16) or quiescent disease (N = 5) and healthy controls (N = 14). Bifidobacteria numbers were significantly reduced in patients (P < 0.01 for active; P < 0.02 for quiescent disease) whereas Bacteroides and Lactobacilli counts remained unchanged. beta-D-Galactosidase activity and Bifidobacteria counts were significantly correlated (P < 0.03). Bifidobacteria are regarded as beneficial for the host. The reduction in Bifidobacteria is responsible for decreased beta-D-galactosidase activity. Thus oral administration of prebiotics that promote their growth might have potential therapeutic interest.