Critically ill neonates have demonstrated low concentrations of the antioxidant glutathione and a high incidence of disease associated with oxidative injury compared to healthy neonates. Cysteine is considered to be a conditionally essential amino acid for neonates and is the rate limiting substrate for the synthesis of glutathione. We hypothesize that parenterally-fed, critically ill neonates administered cysteine supplementation will have higher concentrations of total glutathione, lower ratios of oxidized to reduced glutathione, higher glutathione synthetic rates, lower levels of inflammatory cytokine production and lipid peroxidation, and decreased severity of disease associated with oxidative injury compared to similarly ill parenterally fed neonates without cysteine supplementation. To test this hypothesis, critically ill, parenterally fed neonates assigned randomly to receive a cysteine or an isonitrogenous cysteine-free supplement to their TPN (total parenteral nutrition) regimen will be prospectively studied in a double-blind fashion according to the following specific aims: