A trial was conducted to evaluate the effect of Saccharomyces cerevisiae, live yeast culture (yea sacc(1026)) on protozoa count, percentages of different protozoa types and blood parameters. Three male Kivircik rams with ruminal cannula were used in a Latin square design, during 22 days periods (15 days for adaptation, 7 days for collection). They received 0 (control group), 5 g/ day (i.e.25.10(9) CFU) or 10 g/day (i.e.50.10(9) CFU) of yea-sacc(1026) (assay groups). The cultures were added to the ration with grain diet. Ration was consisted of 70 % grain diet and 30 % alfalfa hay. Rumen contents collected before and 3h after morning feeding on days 1 and 7 in each collection period were analyzed. Blood samples were also collected the same days. Addition of YS to diet has significantly modified the proportions of the different protozoa types, and improved ruminal cellulolytic activity. Moreover, this treatment has also induced systemic biochemical changes: serum total protein, urea, calcium concentrations and Ca/creatinine ratio were increased whereas triglyceride concentrations were lowered. No significant difference in hematological parameters was evidenced, even if white blood cell counts (leukocytes, neutrophiles and lymphocytes) tended to increase and total and corpuscular hemoglobin concentrations to decrease. But the mechanisms of Saccharomyces cerevisiae actions on ruminal metabolic activities, and on lipid, nitrogen and bone metabolisms of sheep remain to be elucidated. The determination of biological consequences (improvement of nutritional status, growth or resistance to infections) requires further investigations.