Unseparated splenocytes (SPCs) or purified SPC subsets from diabetes-prone BB (BBdp) or diabetic BB (BBd) rats were activated in vitro with either phorbol myristate acetate (PMA) and ionomycin (I) or concanavalin A (ConA). Such activated SPCs were then injected intravenously into 30-day-old BBdp rats, and their capacity to induce adoptive transfer (AT) of diabetes was studied. The proliferative response in vitro of BBd unseparated SPCs or purified W3/13+ SPCs (i.e., T lymphocytes + large granular lymphocytes) to PMA + I far exceeded that of ConA, resulting in mean stimulation indices of 68 and 112 (PMA + I) and 1.9 and 30 (ConA). The incidence of AT was similar when equal numbers of unseparated SPCs from the same BBd donor were injected after activation by either PMA + I + interleukin 2 (PII) or ConA (57 vs. 50%, respectively); however, injection of PII-activated and macrophage-depleted W3/13+ SPCs from BBd animals resulted in a significantly higher incidence of AT (90%, P less than 0.05). As few as 0.5 x 10(6) PII-activated W3/13+ SPCs were sufficient to induce AT. Sixteen percent of recipients developed diabetes after injection of activated W3/13+ cells from 40-day-old BBdp donors. To determine which W3/13+ cells might mediate such transfer, purified and PII-preactivated CD4 T lymphocytes from BBd rats were injected, and they succeeded in AT in 44% of the recipients. Preactivated BBd B lymphocytes were unable to induce AT. Although a possible role for large granular lymphocytes cannot be excluded, the results demonstrate that in the BB rat, the beta-cell destruction can be induced by CD4 T lymphocytes.