A mouse anti-rat xenogeneic antiserum, B10.D2 anti-BN, has been found to react with a subpopulation of lymphoid cells of certain mouse strains. The corresponding alloantiserum, B10.D2 anti-B10.BR, reacted in analogous fashion with lymphoid cells of BN rats. In the case of the cross-reaction on mouse cells, mapping studies indicated that at least part of the reactivity was with the product of gene(s) determined by the I-A subregion of the H-2 complex. Chemical isolation studies with radiolabeled cell surface preparations indicated that the antigens detected in both mouse and rat had mol wt characteristic of Ia antigens (35,000 and 28,000 dalton molecules). Testing of fractionated spleen cell populations revealed that the cross-reactive antigens were expressed predominatly on B cells, but that a subpopulation of T cells were also reactive. Wider strain and species distribution studies are in progress to determine the extent of such Ia cross-reactions between species and to further assess the practical and theoretical importance of such cross-reactions.

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