Three experimental manipulations showed the suppressive effect of injection of complete Freund's adjuvant (CFA) in the diabetic state of nonobese diabetic (NOD) mice. (i) Diabetes was inhibited in young NOD mice injected with the adjuvant. (ii) Recurrence of diabetes in mice transplanted with pancreatic islets was also inhibited when the recipients were injected with CFA. (iii) Injection of spleen lymphocytes from diabetic mice into male NOD mice transferred the diabetic state, but this transfer was markedly suppressed when the recipients were treated with CFA. In all three cases the spleen cells from the normoglycemic mice treated with CFA induced diabetes when transferred into NOD male mice. CFA, therefore, induces a state of T-cell dormancy, in which the islets are no longer subject to an immune attack.