Graft-versus-host (GVH) reaction was induced by allogeneic blood lymphocytes in chick embryos and measured by the resulting splenomegaly. Prior to injection into embryos, lymphocytes were incubated in vitro in the presence of complement with rabbit antisera raised against chicken thymus (ATS), bursa cells (ABS), or immunoglobulin (AGS). Selective elimination of GVH-reactive lymphocytes was achieved by ATS but not by ABS. Cross absorption of ATS with bursal cells further increased its specificity. The antiserum concentration was critical: 10% was inhibitory but 1% enhanced the response. Treatment of lymphocytes with AGS under various conditions failed to inhibit the GVH reaction. Treatment of bone marrow cells with ATS eliminated GVH-reactive lymphocytes but preserved the erythropoietic cells. A possible application of this approach to clinical bone marrow transplantation is apparent.