Non-obese diabetic (NOD) mice injected with CD3 antibody as newborns have a reduced incidence of diabetes, raising the possibility that the neonatal injection caused a long-lasting change in circulating T cells. The present study shows that NOD and BALB/c mice injected with soluble CD3 antibody in the first 2 days of life sustained an 80-95% reduction in the number of circulating T cells lasting for 2-3 weeks, with T cells returning after 4 weeks, and reaching control values after 6 weeks. The T cells which appeared in intact mice 4-6 weeks after injection showed no excess of T-cell receptor (TcR) delta expressing cells. They had a similar distribution into CD4 and CD8 subsets as uninjected controls, and a similar usage and cell surface expression of four T-cell receptor V beta families. Labelled CD3 antibody was detected in the serum for up to 2 weeks after injection into neonates and was enriched in the thymus. Adoptively transferred T cells continued to be cleared from the circulation for 4 weeks following antibody injection. The properties of T cells which had been exposed to CD3 neonatally were investigated in animals who were first injected with CD3 antibody and then thymectomized. These animals had reduced numbers of T cells at 12 weeks of age. The surviving T cells showed a Ca2+ flux when stimulated but their proliferation in response to concanavalin A (Con A) was reduced, even in the presence of irradiated accessory cells or T-cell supernatant co-stimulator factors. Although the representation of four different V beta families was the same as in the uninjected controls, the density of expression of the T-cell receptor was reduced. The data indicate that the limited number of T cells which survive the injection are functionally deficient and that an intact thymus is required for full T-cell repopulation following neonatal CD3 injection into NOD mice.