Microbial-derived -galactosidase (-gal) enzyme preparations improvein vivo lactose digestion and tolerance through enhanced gastrointestinal digestion of lactose. Three different -gal preparations, Lactogest (soft gel capsule), Lactaid (caplet), and DairyEase (chewable tablet) and placebo were fed to lactose maldigesters with either 20 g or 50 g of lactose to compare the efficacy of these products and to further establish a dose-response relationship for use. All enzyme preparations dramatically reduced both the peak and total breath hydrogen production when fed with milk containing 20 g of lactose. Four capsules of Lactogest, two caplets of Lactaid, or two tablets of DairyEase (each treatment containing approx 6000 IU) reduced total hydrogen production significantly (P<0.05) below="" that="" observed="" with="" two="" capsules="" of="" lactogest="" (containing="" approx="" 3000="" iu)="" in="" a="" stoichiometric="" manner.="" symptoms="" were="" significantly="">P<0.05) less="" severe="" with="" all="" the="">-gal products. In contrast, with 50 g of lactose in water, peak and total hydrogen production was modestly, but not significantly reduced by the enzyme treatment. Furthermore, symptom scores for bloating, cramping, nausea, pain, diarrhea, and flatus were not different between treatments and the control. The 50-g lactose dose appeared to overwhelm the ability of either 3000 or 6000 IU of -gal to assist significantly with lactose digestion. Results from these studies demonstrate the relative equivalency of chewable, caplet, and soft-get -gal products, based on IUs of enzyme fed.