Well, appropriate-for-gestational age, low-birth-weight infants weighing 2,100 gm or less were divided into three gestational age groups and assigned randomly within each age group to one of five feeding regimens: pooled human milk; formula 1 (F1) = 1.5gm/dl protein, 60 parts bovine whey proteins: 40 parts bovine caseins; F2 = 3.0 gm/dl, 60:40; F3 = 1.5 gm/dl, 18:82; F4=3.0 gm/dl, 18:82. Plasma and urine concentrations of methionine and of cystathionine were higher in the infants fed F1 to F4 than in the infants fed BM. The plasma cystine concentrations of infants fed F2 (which had a cystine content at least twice that of any of the other formulas) were significantly higher than those of infants fed BM. Plasma taurine concentrations of infants fed F1 or F4, which were virtually devoid of taurine, decreased steadily during the course of study becoming lower than those of infants fed BM. Urine taurine concentrations of infants fed F1, F3, or F4 (but not F2 which had more taurine than F1, F3, or F4) were lower than those of infants fed BM. These results provide further evidence for the limited capacity of the preterm human infant to convert methionine to cystine, owing to delayed maturation of cytathionase, and suggest a limited capacity to convert cystine to taurine. The latter suggestion is consistent with low human hepatic cysteinesulfinic acid decarboxylase activity 0.26 (fetal) and 0.32 (adult) nmoles/mg protein/hour vs 468 in rat liver.