In a previous study,1phenylalanine (L-phenylalanine) was found to cause an inhibition of the immune response to diphtheria toxoid in rabbits and rats. Because phenylalanine interferes with the in vitro uptake of histidine, ornithine and tyrosine by the brain,2it appeared that phenylalanine may inhibit antibody synthesis by limiting the uptake and intracellular availability of essential amino acids. A reduction in availability of amino acids may be the ultimate cause of the decrease of antibody synthesis. The purpose of the present study was to analyze possible mechanisms of phenylalanine inhibition of the immune response and to determine if phenylalanine will inhibit the homograft reaction. Materials and Methods Amino Acid Analyses.— Twenty C57/HeJ mice were injected intraperitoneally with phenylalanine, 100 mg/day, for four days. The mice were killed by decapitation three hours after the last injection. The spleens were removed immediately, weighed, and frozen on solid carbon dioxide. The