The serologically detectable (Ag-B, H-1) histocompatibility antigens and mixed lymphocyte responses of geographically widespread populations of wild rats (Rattus norvegicus) were examined with reagents and cells derived from standard inbred rat strains. The histocompatibility antigens of the vast majority of wild rats from France, England, and the United States were very similar, if not identical, to those present in the inbred strains. All but one out of 89 animals displayed one or two Ag-B specificities detected by operationally monospecific alloantisera against Ag-B haplotypes 1 through 8 with the exception of the Ag-B4 group. This group was shown to include the Ag-B4 (H-1a), Ag-B9 (H-1d), and Ag-B10 (H-1f) haplotypes.

The mixed lymphocyte response (MLR) of peripheral blood lymphocytes from individual wild rats was established by testing against irradiated splenic lymphocytes from inbred rat strains representing seven MLR types. Identity reactions against one, and in many cases two, inbred MLR types were detected in 54 of 59 wild rats successfully tested. Two-way MLR testing established that the identity reactions between wild and inbred animals were reciprocal and that identity reactions could be shown to exist between wild rats from widely separated geographical locations.

The frequency of individual Ag-B(H-1) and MLR phenotypes was clearly different in rats from separate geographical locations. The English population was the most homogenous with only one Ag-B(H-1) and two MLR phenotypes, whereas the Pittsburgh population was more heterogenous and contained seven Ag-B(H-1) and three MLR phenotypes. Although occasional animals could not be characterized for their Ag-B or MLR type, the number of such animals was small, indicating that the degree of genetic polymorphism in the loci controlling these phenotypes in rats is restricted.

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