In previous studies we demonstrated that an induced asialo-GM1 positive (ASGM1+) cell of donor origin that exerts natural killer cell-like activity (NK activity+) plays a crucial role in the development of graft-versus-host (GVH)-associated tissue damage and severe immunosuppression. This study examined whether the ASGM1+ (NK activity+) GVH effector cells were activated by non-specific signals or whether these cells were triggered by specific alloantigens and displayed antigenic specificity. C57B1/6 (B6) donor mice were treated with either B6 x AF1 (B6AF1) lymphoid cells and anti-asialo GM1 antibodies (anti-ASGM1) to induce and eliminate specifically activated B6-anti-B6AF1 ASGM1+ (NK activity+) cells or with polyinosinic: polycytidylic acid (poly I:C), and anti-ASGM1 to eliminate non-specifically activated ASGM1+ (NK activity+) cells. Donor spleen and lymph node cells depleted of the specific allo-induced ASGM1+ NK reactive cells showed near normal numbers of L3T4+ and Lyt-2+ cells and retained T- and B-cell functions as measured by mitogen responses (to PHA, Con A and LPS), mixed lymphocyte responses (MLR) (to B6AF1) and the generation of cytotoxic T cells (CTL) (to B6AF1 blasts). Anti-ASGM1 treatment almost completely abrogated NK activity in all donor inocula. GVH reactions were induced by injecting treated donor cells into B6AF1, B6 x C3HejF1 (B6C3HF1) and B6 x SJLF1 (B6SJLF1) hybrids and monitored by splenomegaly, suppression of T-cell mitogen responses and the development of histopathological lesions in the thymus, liver and pancreas. Cells from donors depleted of non-specifically (poly I:C) induced ASGM1+ cells induced severe histological lesions, marked immunosuppression and splenomegaly in all three F1 hybrid combinations. When the donor cells were depleted of specifically induced (B6-anti-B6AF1) ASGM1+ cells and injected into the three F1 combinations they induced splenomegaly in all three but caused severe tissue injury and intense immunosuppression only in B6C3HF1 and B6SJLF1 mice and not in B6AF1 mice. Genetic analysis suggests that the H-2D (or a closely related) region of the H-2 complex plays an important role in the activation of the specific GVH effector cells. These results suggest that the cell(s) responsible for splenomegaly are different from the ones that cause severe GVH-associated tissue damage and immunosuppression although there may be cells and/or lymphokines common to both processes.(ABSTRACT TRUNCATED AT 400 WORDS)