The effects of supplementing total parenteral nutrition (TPN) solutions with cysteine were assessed at two different levels of nitrogen intake by determining nitrogen retention, sulfate excretion, and sulfur-containing amino acid concentrations. Ten infants received 72 mg/ kg/day of cysteine-HCl in a TPN solution for a period of 6 days. Five of these infants received 251 ± 48 (±SD) mg/kg/day of nitrogen, and five received 403 ± 45 mg/ kg/day of nitrogen. Two other groups of five infants each received unsupplemented TPN at nitrogen intakes of 235 ± 48 and 412 ± 54 mg/kg/day, respectively. Fluid and nonprotein caloric intakes were similar for all four groups. Cysteine supplementation increased plasma and urine free cyst(e)ine concentrations and enhanced total sulfur retention, but did not enhance nitrogen retention. [Cyst(e)ine refers to the mixture in any proportion of the sulfhydryl (cysteine) and the disulfide (cystine) forms of this compound.] Nitrogen retention, sulfate excretion, cyst(e)ine excretion, and plasma taurine concentrations increased as the result of the increase in nitrogen intake.