To investigate the possible causes of relatively low blood .gamma.-globulin levels (.ltoreq. 0.5 gm/dl) during the 1st yr of life, 287 patients less than 1 yr of age who were suffering from mild diseases were studied retrospectively. They were divided into 2 groups, those with a .gamma.-globulin level < 0.5 gm/dl and those with a .gamma.-globulin level > 0.5 gm/dl. By reconstructing the diets given, it was found that they had been receiving isocaloric diets that differed only in protein supply: patients with lower .gamma.-globulin levels received 2.9 gm/kg per day of protein and patients with higher .gamma.-globulin levels received 4.0 gm/kg per day. Healthy subjects (55) were studied prospectively during the 1st yr of life on 2 isocaloric diets that differed only in their protein content: the 1st one supplied 2.5 gm/kg per day of protein and the 2nd supplied 4.0 gm/kg per day. At approximately 5, 7 and 10 mo. of age, Hb, total protein, albumin, globulins, immunoglobulins and other common blood parameters were measured. Patients on the lower protein diet had a .gamma.-globulin concentration of less than or equal to 0.5 gm/dl and those on the higher protein diet had a .gamma.-globulin concentration of .gtoreq. 0.8 gm/dl. Immunoglobulin levels, particularly Ig[immunoglobulin]G, were lower in patients on the lower protein diet. The subjects with lower protein intake and lower levels of .gamma.-globulin and immunoglobulins showed significantly higher morbidity.