We have investigated whether suppression of the allogeneic response after blood transfusion (BT) could be due to inhibition of the production or activity of Interleukin-2 (IL-2), a soluble mediator involved in T lymphocyte proliferation. Reduction of IL-2 production after BT was less frequent than, but significantly associated with, non specific MLR suppression. However, MLR suppressor cells did not inhibit the release of IL-2 from autologous pre-BT lymphocytes. In addition, post-BT soluble suppressor factors of the MLR did not affect the ability of IL-2 to promote cell proliferation. Thus, although MLR suppression after BT is associated with reduced IL-2 production, this is not a major mechanism involved in this effect.