Forty percent dietary restriction on 9-weeks-old C3H/He mice caused decrease of the weight of central lymphoid organs in parallel with the reduction of body weight. However, the percentage of splenic T cells was dramatically increased in diet-restricted mouse spleen cells. Generally, normal mouse spleen cells contained about 30% of Thy 1.2+ T cells, but the restricted mouse spleen cells contained 80% Thy 1.2+ T cells. Ly 1+, L3T4+ T cells, but not Ly 2+ T cells, also increased in diet-restricted mouse compared with the unrestricted mice. In parallel with the dramatic changes of splenic T cells, spleen cells obtained from diet-restricted mice showed higher immunological responses against alloantigen and interleukin 2. It was also demonstrated than nylon-passed splenic T cells obtained from diet-restricted mice showed higher levels of T cell responses against r-IL-2 and alloantigen, indicating that dietary restriction modulates T cell functions themselves.