The mouse alloantiserum B10.D2 anti-B10.BR (H-2d anti-H-2k) crossreacted with rat lymphocyte surface glycoproteins with characteristics of Ia antigens. Sequential precipitation analysis on solubilized radiolabeled Lewis rat spleen cell alloantigens pretreated with BN anti-Lewis (Ag-B3 anti-Ag-B1) alloantiserum revealed that the Ia-like antigens detected by the mouse alloantiserum also reacted with the rat anti-Ia antibodies. The rat alloantiserum also detected another set of Ia-like antigens which did not crossreact with the mouse alloantibody. Precipitation analysis using congenic rat strains confirmed that all Ia-like antigens precipitated by the rat alloantibody were encoded by Ag-B-linked genes. Thus the shared Ia-like antigen must also be the product of Ag-B-linked gene(s) or be physically associated with such products. The existence of two separable Ia antigens suggest the existence of more than one sublocus coding for Ia antigens within the rat MHC. The determinant detected on the crossreactive molecule by the mouse alloantibody is probably distinct from that detected by the rat alloantibody since the mouse alloantibody reacted similarly with both Lewis and BN lymphocytes.