SUMMARY Uptake of 3H-thymidine was used to quantify the reactivity of immunocompetent cells from AKR, CBA, A, C57BL/6, or DBA/2 mice following culture with mitomycin C-treated AKR cells. Median survival times and percentage of survival at 100 days were used to quantify the severity of graft-versus-host (GVH) disease in immunosuppressed AKR mice that were given immunocompetent cells from AKR, CBA, A, C57BL/6, or DBA/2 mice. A correlation was suggested when the results of the one-way mixed leukocyte cultures and the scores of GVH reactivity in vivo were compared. AKR mice bearing a long passage lymphocytic leukemia were treated with total body X-irradiation, cyclophosphamide, and adoptive immunotherapy. A bioassay was employed to quantify the graft-versus-leukemia (GVL) reactivity of immunocompetent cells from AKR, CBA, A, C57BL/6, or DBA/2 donors. There was an apparent correlation between GVH and GVL reactivity of immunocompetent cells from AKR, CBA, and C57BL/6 mice. However, no correlation was found in the GVH and GVL reactivity of immunocompetent cells from A or DBA/2 donors. Cells from strain A donors caused the most severe GVH disease, but had only moderate antileukemic effect. The most important finding was that cells from DBA/2 donors caused somewhat less severe GVH disease, but had the highest GVL reactivity.