The synthesis of essential amino acids by the gut microflora of pigs, and their absorption, were assessed from the incorporation of (15)N from dietary (15)NH(4)Cl and of (14)C from dietary (14)C-polyglucose into amino acids in the body tissues of four pigs. Both (15)N and (14)C were incorporated into essential amino acids in body protein. Because pig tissues cannot incorporate (15)N into lysine or (14)C into essential amino acids, the labeling of these amino acids in body protein indicated their microbial origin. The absorption of microbial amino acids was estimated by multiplying the total content of each amino acid in the body by the ratio of the isotopic enrichment of the amino acid in the body to that in microbial protein. Because the ratio of (14)C:(15)N in body lysine was closer to that in the microflora of the ileum than to that of the cecum, absorption was assumed to take place exclusively in the ileum. The estimates of microbial amino acid absorption from (14)C-labeling were as follows (g/d): valine 1.8, isoleucine 0.8, leucine 2.0, phenylalanine 0.3 and lysine 0.9, whereas for lysine, the estimate from (15)N-labeling was 1.3 g/d. We conclude that the gastrointestinal microflora contribute significantly to the amino acid requirements of pigs.