Abstract. Alloreactive cytotoxic antibodies were induced in BALB/c mice by syngeneic immunization with normal lymphoid cells. Sixteen out of 41 mice produced antibodies with distinct anti-H-2 specificity. Anti-Kk antibodies were present in all positive sera, but the individual sera produced different reactivity patterns when tested on a panel of H-2 haplotypes. Absorption and immunoprecipitation experiments confirmed the H-2 specificity of the syngeneic sera. We hypothesize that virus-modified H-2d structures have triggered alloreactive B-cell clones to produce anti-H-2 antibodies.