A study was conducted on 28 cows between the third week preceding and fourteenth week after calving. The cows were divided into 4 groups of 7. The animals in the control group (C) were fed a basic ration consisting of maize and beet top silage, meadow hay, soyabean oilmeal, concentrate for extra milk production and mineral mixtures according to the INRA system. This ration was supplemented for the experimental groups with Yea-Sacc preparation (Y) or with preparations containing baker's yeast Saccharomyces cerevisiae G1 produced according to a cell-engineering method (X) or Saccharomyces carlsbergensis brewery strain SK-1 (S). Average milk production during the experimental period in the respective groups was, in kg: 26.7 - C; 28.2 - X and S; 27.7 - Y; the milk-fat content: 4.05; 4.01; 3.99 and 4.29% and protein content, 3.01; 2.98; 3.09 and 3.15%. The persistence of milk yield after the peak of lactation was greater in cows receiving a yeast supplement than in the cows of the control group.