We studied the effects of anti-CD2 monoclonal antibodies (MAb) on spontaneous and induced autoimmune diabetes mellitus in diabetes-prone (DP) and diabetes-resistant (DR) BB/Wor rats. In DP rats, all anti-CD2 MAb prevented spontaneous diabetes and the adoptive transfer of diabetes with Con-A--stimulated acute diabetic spleen cells; OX34 prevented Poly I:C induced accelerated onset of diabetes and the adoptive transfer of diabetes with Con-A--stimulated RT6.1+ T cell depleted DR splenocytes. In DP rats, all anti-CD2 MAb except OX53 depleted CD4+ T cells, without depleting natural killer cells or CD8+ T cells. OX34 injected DR rats were profoundly depleted of CD4+ T cells without evidence of decreased CD8+ T cells, but were not protected against the induction of diabetes by RT6.1+ T-cell depletion and Poly I:C injections. We conclude that anti-CD2 MAbs protect against BB/Wor autoimmune diabetes by depleting CD4+ T cells, preventing the activation of effector cells, or by blocking CD2/ligand interaction between effector and target cells.