An experiment lasting 73 days was carried out on 40 Black-and-White Lowland 7-day-old calves divided into 5 groups of 8. The animals received 8 kg of whole milk daily for 5 weeks, 6 kg in the 6th and 3 kg in 7th week with free access to concentrate mixture and meadow hay. After 7 weeks of the experiment the calves were fed only concentrate to appetite and 0.3-0.4 kg of meadow hay. This basic control diet was supplemented with wort without yeast and in the experimental groups with liquid cultures of the yeast, Saccharomyces cerevisiae 1026 or Saccharomyces carlsbergensis of the brewery strain SK-1, BS-Bratislava or wine strain T-81. All strains of yeast significantly stimulated amylolytic and proteolytic activity of the digesta in the duodenum, small intestine and ileum but not in the abomasum. Body weight gains of calves fed diets supplemented with SK-1 and B-Bratislava yeast were higher than in control or in groups supplemented with the other two yeast strains.