1. An intestinal perfusion technique has been used to study absorption in vivo of l-cystine and l-cysteine in six normal human subjects and three patients with homozygous cystinuria.
2. No significant absorption of l-cystine was detected during perfusion of the cystinuric patients, whereas l-cysteine absorption was normal. These results imply that l-cystine and l-cysteine are normally absorbed by different transport processes.
3. No significant reduction of l-cystine to l-cysteine occurred in the gut lumen during the perfusion experiments. No more oxidation of l-cysteine to l-cystine occurred in the gut lumen during the perfusion experiments than in the test solutions which were simultaneously incubated in vitro.