Transfusions of spleen cells are known to prevent spontaneous autoimmune diabetes in susceptible BB/W rats, while T cell-depleted transfusions are ineffective. To characterize further the protective cell(s), we transfused young diabetes prone rats with splenocytes from diabetes resistant BB/W rats that were treated in vitro to enrich them in either OX8+ (suppressor/cytotoxic) T cells or W3/25+ (helper/inducer) T cells. Diabetes subsequently occurred in 19 of 29 (66%) recipients of OX8-enriched, W3/25-depleted cells and 20 of 37 (54%) controls, but in only 7 of 30 (23%) recipients of W3/25-enriched, OX8-depleted cells (p less than 0.005). Transfusion of spleen cells from diabetes resistant donor rats pretreated in vivo to deplete OX8+ cells also prevented diabetes in susceptible BB/W recipients. We conclude that transfusions of W3/25+ helper/inducer splenic T lymphocytes obtained from diabetes resistant animals prevent spontaneous diabetes in the BB/W rat.