Eight early lactation, primiparous Holstein cows were fitted with ruminal and duodenal cannulas. The cows were used in a replicated 4 x 4 Latin square design to test the effects of yeast culture (0 vs. 10 g/ d) and dietary crude protein (CP) (16.1 vs. 18.8% of dry matter) in 44% forage diets. Dietary CP differed primarily in ruminally degradable CP (9.1 vs. 11.4% of dry matter). Dry matter intake tended to increase as amount of yeast culture increased. However, yeast culture had no effect on ruminal pH, concentrations of NH3 and volatile fatty acids in ruminal fluid, or ruminal digestibility. Yeast culture increased the ruminal concentration of isobutyrate in cows fed the low CP diet and decreased the concentration of isobutyrate in cows fed the high CP diet. The higher CP diets increased microbial N passage to the duodenum and had no effect on passage of nonmicrobial nonammonia N. Flows to the duodenum of nonmicrobial nonammonia N tended to be higher for cows fed yeast culture. Flows of essential amino acids to the duodenum and the essential amino acid profiles of duodenal digesta and of mixed ruminal bacteria were not altered by yeast culture. Yields of fat and 4% fat-corrected milk were increased by yeast culture supplementation of the low CP diet. Similar tendencies were noted for yields of milk and milk protein.