Autoimmune diabetes was induced with an established model in which 3 daily injections of 95 mg/kg body wt/day streptozocin (STZ) and 2 x 104 U interferon-γ (IFN-γ) were administered to C57BL/6 mice. Diabetes onset was accompanied by precipitous increases in serum glucose levels and validated by immunoperoxidase studies showing diminished islets in pancreatic tissue sections. Administration of two to three doses of a monoclonal antibody (MoAb) or an immunotoxin (IT) directed against the CD3 ε-chain before STZ/IFN-γ treatment prevented increases in serum glucose and protected islets from damage. IT was made by crosslinking anti-CD3 to a low oligosaccharide-containing fraction of purified ricin toxin A chain (RTA; a catalytic inhibitor of protein synthesis) with a stabilized derivative of 2- iminothiolane. Protection was complete, long-lived, and selective because two different control ITs did not prevent diabetes onset. A second pan T-cell- reactive IT was synthesized by linking the MoAb anti-Ly1 to the same RTA toxin. Anti-Ly1 reacts with the murine homologue of human CD5. Anti-Ly1 RTA also protected against diabetes onset in a dose-dependent manner requiring higher doses and a longer schedule than anti-CD3 or anti-CD3 RTA. These studies demonstrate for the first time the importance of CD3+ and CD5+ cells in diabetes onset in the low-dose STZ/IFN-γ model and show that anti- CD3, anti-CD3 RTA, or anti-CD5 RTA may be useful in vivo for the treatment of diabetes or perhaps other T-cell-mediated autoimmune diseases. These data may have important therapeutic implications for early autoimmune diabetes in humans.