Supplementation with alanine was found to increase growth in weight and nitrogen balance in 5 infants with a variety of inborn errors of amino acid metabolism receiving diets restricted in protein. The addition of alanine to the regimen led to a mean increase in weight of 15 g/day. This and the increased nitrogen balance of 15 mg/kg/day were highly significant statistically. In addition a dose-response effect of alanine was observed. The effect of alanine was compared with that of a supplemental mixture of essential and non-essential amino acids, lacking only those considered to be toxic in these patients. Alanine at 0.05 g/kg was as effective in promoting growth in weight as 1.05 g/kg of the amino acid mixture, while 0.25 g/kg of alanine was more effective than 0.70 g/kg of the amino acid mixture. The protein sparing anabolic effect of alanine is thought to be a reflection of the alanine glucose cycle.