An experiment was conducted to characterize the effects of postruminal administration of casein, glutamine, cornstarch, and water on protein turnover and in vitro muscle cell proliferation. Four MARC III steers (205 kg) were fed a protein-restricted bromegrass hay-based diet (2.86 Mcal of DE/kg and 13.6 g of N/kg). Using a 4 x 5 Latin square arrangement balanced for residual effects, casein and glutamine, equal to 50% of basal dietary nitrogen intake, cornstarch, isocaloric with casein infusion, or an equal volume of water was continuously infused into the abomasum of steers. Blood samples, collected every 2 h for 24 h after 7 d of infusion, were tested for the effect on cell cycle kinetics and myotube protein turnover. Urine and feces were also collected for 4 d after blood sampling for nitrogen balance and fractional skeletal muscle degradation. The mitogenic activity and ability of serum to influence rate of myoblast proliferation in a dose-dependent manner was influenced (P < .05) by infusate: casein > cornstarch > glutamine = water. Abomasal infusion of casein and cornstarch increased (P < .05) in vitro muscle protein synthesis and decreased (P < .05) in vitro muscle protein degradation, whereas abomasal glutamine infusion only increased (P < .05) in vitro muscle protein synthesis. Abomasal glutamine infusion decreased (P < .05) fractional skeletal muscle protein degradation and synthesis; however, fractional muscle protein accretion tended to increase due to a greater decline in fractional muscle protein degradation. In contrast, abomasal casein infusion increased (P < .05) fractional skeletal muscle protein synthesis, breakdown, and accretion. These results suggest that muscle hypertrophy may be regulated by serum constituents whose activity is affected by postruminal amino acid flow.