Effects of chronic dietary protein restriction on immunological response were studied in BALB/c male mice. Three diets were used: 4, 8, 24% protein (casein) which 1000 mice were allowed to eat ad libitum. Restricted mice were not malnourished. Mice subjected to 4% protein diet had life expectancy that was marginally significantly prolonged when compared with "control" mice eating 24% protein diet. In vivo primary antibody responses to sheep erythrocytes, assessed by plaque formation and serum hemagglutination, were lower in 4% protein fed mice than in those eating 24% protein diets. Primary antibody responses fell with age in all diet groups. In vitro splenic lymphocytic blastogenesis, measured by 3H-thymidine uptake to concanavalin A, phytohemagglutinin, lipopolysaccharide, and allogenic lymphocytes, was not significantly influenced by diet. Thymus weight was not importantly influenced by diet. Spleens from mice eating 4% protein diet and immunized with sheep erythrocytes were significantly smaller than those from control animals. Our findings are consistent with several other reports, and indicate that this type of diet restriction does not delay onset of aging patterns found in the immune system.