The paper presents the results of clinical and laboratory studies of lactrase, a drug containing lactase. This agent is recommended for splitting lactic sugar in subjects with appreciably decreased production of endogenous lactase (hypolactasia). Twenty-eight patients with this condition were examined. Manifest clinical symptoms of the condition were observed after loading with 50 g of lactose in all examinees. Addition of 250 mg of lactrase to lactose led to complete clinical compensation of the deficit of endogenous lactase in 75% examinees, and if 500 mg of lactrase was administered, hypolactasia was compensated in virtually all patients. A single intake of 50 g of lactose with lactrase causes a statistically reliable increase of glycemia in such patients. Moreover, a reliable effect of lactrase was observed when measuring galactose in the urine following the lactose test with 250 and 500 mg of lactrase. Our results indicate a high efficacy of lactrase in the treatment of patients with hypolactasia.