The mixed leucocyte reaction has been studied using lymphocytes from three inbred rat strains and their F1hybrids. The results indicate that the reaction in these strains detects histoincompatibility only at the strongH-locus(Ag-B).Analysis of the results suggests that antigens determined byAg-Balleles vary in the strength of reaction they elicit from histoincompatible lymphocytes or that lymphocytes from rats of different strains vary in intensity of reaction to the antigen.