AbstractAn immunoregulatory circuit is described in which B cell blasts activate syngeneic Ly‐1+2−3−T cells to (a) start a reaction which is indistinguishable from a graft‐vs.‐ host reaction (syngeneic GvH) and (b) induce suppressor cell activity which abrogates the syngeneic GvH. Since capping the surface immunoglobulin (Ig) on B cell blasts blocks their ability to activate this circuit, it is likely that the relevant cell surface structure “seen” on the B cell by the Ly‐1 T cell is either Ig itself or another molecule in asso