The use of direct fed microbials has been shown to enhance digestion in the ruminant. One source of microbial live populations is a yeast culture Saccharomyces cerevisae (Yea-Sacc1026, CBS 493.94; Alltech Inc, USA), and its use in ruminants has been associated with a range of benefits including an overall increase in dry matter and fiber digestibility (Wiedmeier and Arambel, 1985), combating heat stress in lactating cows (Huber and Higgenbotham, 1985) and increased performance (Fallon and Harte, 1987). Glade and Biesik (1982) working with yearling horses, demonstrated increased DM and N digestibility when S. cerevisae was added to the diet. It was suggested that S. cerevisae enhanced cellulolytic activity, triggering microbial metabolism changes in the horse’s large intestine that enhance hemicellulose fermentation. They also reported that the addition of S. cerevisae results in smaller percentages of absorbed nitrogen excreted in the urine, indicating that the biological value of the S. cerevisae -supplemented diets may be more than the direct contribution of the nitrogen within the yeast itself. The aim of this review is to determine the effects of S. cerevisae on performance parameters of bulls.