The absorption of lysine synthesised by the gastrointestinal microflora was estimated by comparing the 15N incorporated into body lysine in four germ-free (15N-GF) and four conventional (15N-CV) rats. They were fed for 10 d on a protein-free diet containing fermentable carbohydrates and 15NH4Cl; another four conventional rats (control), fed on the same diet but with unlabelled NH4Cl, were used to estimate the natural abundance of 15N. The eviscerated carcass of each rat was homogenized and a sample hydrolysed. Lysine was isolated by ion-exchange chromatography and its 15N enrichment was measured by isotoperatio mass spectrometry. The 15N-CV rats significantly incorporated 15N into their body lysine. The 15N-GF rats had a statistically significant, although small, incorporation of 15N into their body lysine, probably arising from a measurement artifact. It was concluded, therefore, that all [15N]lysine was of microbial origin. The total lysine content in the body and the 15N enrichment of lysine in the microbial fraction of the faeces of the 15N-CV rats were also determined. The amount of microbial lysine absorbed by the 15N-CV rats was estimated by dividing the total amount of [15N]lysine in the body by the enrichment of microbial lysine. It was estimated that the daily absorption of microbial lysine by the conventional rats was 21.3 (SE 2.04) mg/kg body weight0.75.