We previously demonstrated in the (Lew.1W----Lew.1A) congeneic strain combination differing only at the RT1 Region that blood transfusion sharing RT1.B with the heart donor lead to longterm graft function. In the present paper, using the same genetic combination and limiting dilution analysis we studied the frequency of recipient splenocytes (Lew.1A) able to produce IL2 upon donor peritoneal macrophages challenge. We found that splenocytes of Lew.1A rats, previously transfused with Lew.1W blood and grafted with Lew.1W heart, contained suppressor cells which inhibited the capacity of autologous lymphocytes to produce IL2 upon Lew.1W challenge. When a Lew.1A recipient received either a Lew.1W heart without previous transfusion or a Lew.1W blood transfusion without heart graft there is no evidence of generation of suppressor cells. This suggests that both blood transfusion and allograft are required for IL2 suppression and that this suppression may be related to the heart tolerance.