Donor-specific transfusion (DST)-induced immunosuppression plays a significant role in clinical and experimental transplantation. To clarify the mechanism of suppression on alloreactivity the suppressor cell induction and the non-cytotoxic blocking antibody production and importance was studied in 15 healthy volunteers and 3 kidney transplant recipients (KTR) after DST on mixed lymphocyte culture (MLC). Significant decrease of anti-donor MLC response was found in all of KTR and in 12 cases of the 15 transfused volunteers. Of 18 cases, 15 had blocking factors in their post-DST serum which strongly (52-91%) inhibited the MLC response. The IgG fractions isolated from the "blocking sera" were responsible for this inhibition. Eighty-one percent of non-cytotoxic blocking antibodies affected the responder cells in MLC and reacted with third party responder cells as well. Both buffy coat and platelet transfusions evoked production of the non-specific blocking antibodies. Our data strongly suggest that DST induces not only the differentiation of suppressor cells and production of anti-idiotypic antibodies, but also the appearance of non-specific, non-cytotoxic antibodies, which may participate in the cell-mediated immune suppression of the alloimmune reactivity.