Lactase-deficient subjecs more effectively digest lactose in yogurt than lactose in other dairy products, apparently due to yogurt microbial β-galactosidase (β-gal) which is active in the GI tract. We evaluated the effects of buffering capacity of yogurt, gastric pH, and microbial cell disruption on β-gal activity and lactose digestion. Three times more acid was required to acidify yogurt than to acidify milk. Yogurt β-gal was stable at pH 4.0 but inactivated at lower pH. When yogurt was sonicated to disrupt microbial cell structure, only 20% activity remained after incubation at pH 4.0 for 60 min. In vivo gastric pH remained > 2.7 for 3 h after ingestion of yogurt. Acidified milk alone or with disrupted yogurt microorganisms caused twice as much lactose malabsorption as did acidified milk containing intact yogurt microorganisms. The results provide a possible explanation for the survival of β-gal activity from yogurt in the GI tract.