Protein-calorie malnutrition leads to depression of host cell-mediated immunity. Nutritional repletion initially results in rapid weight gain followed by a more gradual return of immunocompetence. Administration of a synthetic amino acid diet to normal animals did not preserve body weight or cell-mediated immunity any better than did a high carbohydrate, protein-free diet. Administration of a synthetic amino acid diet to malnourished animals maintained body weight, but did not restore immunocompetence. Proper nutritional repletion should provide both adequate protein and nonprotein calories if a return of immunocompetence is to be anticipated.