This is the first part of a research report on the in vivo properties of lysosubtilin, a broad-spectrum preparation of lytic enzymes from Bacillus subtilis designed for veterinary medicine. This preparation has both antimicrobial properties and the capacity to compensate for digestive enzyme deficiencies in newborn calves. This study demonstrates its efficacy for both the prophylaxis and treatment of calf gastrointestinal disorders. Dose determination studies involved calves fed with colostrum or later milk, supplemented with 10 g/L of sodium bicarbonate, which received either a prophylactic dose of lysosubtilin of 2 x 10(4) U/kg of weight given twice daily for 10 days beginning with the first feeding (30-45 min after birth) or a therapeutic dose of 2.5 x 10(4) U/kg of weight given twice daily until recovery. These could be used for the prophylaxis and treatment, respectively, of the disorders mentioned above with an efficacy up to 100%. Trials in veterinary medicine were carried out in four former Soviet Union republics. More than 16,000 newborn calves were involved. The efficacy of prophylaxis and treatment was found to be 94.4-100% and 95.7-100% respectively, while the efficacy of regular methods was no higher than 82.0 and 91.8%, respectively. A positive impact of lysosubtilin on the improvement of digestive tract functioning, development of an animal and its protective functions against disease was also demonstrated. The consequent increases in calves' daily weight gain, abomasal proteolytic capacity as well as in amount of gamma-globulin, lysozyme and bactericidal activities in their blood sera were significant.