The effect of microbial beta-galactosidase (EC 18.104.22.168) activity on intestinal lactose digestion was estimated directly by following post-prandial venous plasma galactose concentrations. To avoid superimposing effects of free galactose, as with yogurt, fresh or heat-treated suspensions of mechanically disintegrated kefir grains in kefir, containing lactose but no free galactose, were fed to ten Göttingen minipigs. Each meal contained 101.1 (SEM 0.1) mmol lactose in kefir supplemented by either native or heat-treated kefir grains corresponding to a mean beta-galactosidase activity of either 72 (SEM 8) U or zero. Feeding kefir with beta-galactosidase activity resulted in a 30% enhancement of the mean post-prandial plasma galactose peak concentration from 33 (SEM 7) to 43 (SEM 12) mumol/l (n 10), as well as in 23% greater mean areas under the galactose-response curves (8.1 (SEM 1.5) v. 6.6 (SEM 1.2) mmol/min per l) if compared with kefir with heat-treated grains. Both differences were significant (P less than 0.05; paired Wilcoxon test by ranks). There was no induction of intestinal beta-galactosidase (EC 22.214.171.124) activity or intestinal lactose-hydrolysing bacteria by lactose feeding. These results give direct evidence of an enhanced lactose digestion and absorption in native fermented milk products due to the microbial beta-galactosidase activity.