Abstract
The mixed leukocyte reaction was studied using lymphocytes from 3 inbred rat strains and their F1 hybrids. The results in-dicate that the reaction in these strains detects histoincompatibility only at the strong H-locus (Ag-B). Analysis of the results suggests that antigens determined by Ag-B alleles 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.