Lymphocyte subsets from four groups of Lobund-Wistar (L-W) rats were quantitated to determine the influence of diet restriction (DR) and exposure to environmental antigens on the development of these cells. The following effects were found from 6 to 30 months of age: The number of Ts/c and Ts cells were higher in germ-free (GF) vs conventional (CV) rats, whereas B-cell numbers were lower. W3/13 T cells, Ts, and NK cell numbers were higher in DR vs full-fed rats, whereas B-cell numbers were lower. OX19 and W3/13 T cell numbers decreased from 6 months to 30 months in each group, whereas NK cell numbers increased. Also, OX6+ B cell numbers increased with age, and Ts/c numbers decreased. These data may reflect a relationship between enhanced T-cell function and the extended life span and lower tumor incidence observed in DR L-W rats.