A yeast culture preparation, Yea-Sacc¹⁰²⁶ (Alltech, Inc.), was introduced into dairy cow rations at 10 g/d over the winter period to determine its effect on cow productivity. Twenty-four Holstein-Friesian x Black-and-White dairy cows were divided by the analog method into two groups of 12 cows each. The test was carried out over the period from 5 to 85 days following calving. Fat corrected milk yield was significantly higher (P<0.05) in the experimental than in control group (16.5 kg vs 15.6 kg/d). Milk fat percentage and milk protein content increased by 4.5% and 2.9% (P<0.05), respectively in the group fed viable yeast culture. Yeast culture (YC) supplementation increased fat yield (P<0.01) and protein yield (P<0.05) by 7.8% and 6.1%, respectively.