Abstract Washed cell suspensions of mixed rumen bacteria were used to evaluate effects of 100% urea-nitrogen and 75% urea-nitrogen plus 25% amino acid-nitrogen in growth media upon microbial growth rate and yield, specific rate of glucose consumption, and incorporation of glucose into mixed cells, carbon dioxide, and end products. Rumen microbial dry matter, nitrogen, ribonucleic acid, deoxyribonucleic acid, glucose disappearance, and production of volatile fatty acids were considerably higher in medium containing urea plus amino acids as compared with urea only. Specific growth rates of microbes were .104 and .203 and mean doubling times were 6.7 and 3.4h in the urea and urea plus amino acid growth media. Microbial growth in mg per 100mg glucose used, per mole glucose and per mole adenosine triphosphate (ATP), and specific rate of glucose consumption in mmol per mg cells·h were 19.3, 34.7, 15.4, and .016 with urea, and 24.4, 44.2, 20.6, and .014 with urea plus amino acids. Percentages of catabolized glucose incorporated into microbial cells, carbon dioxide, and end products did not differ between treatments and averaged 19.5, 7.8, and 64.4%.