A rat Ia (RT‐1B) antigen (called Ia‐A) equivalent to the mouse I‐A product has been defined with mouse monoclonal antibodies (W. R. McMaster and A. F. Williams, Eur. J. Immunol. 1979. 9: 426). To identify other Ia antigens mouse monoclonal antibodies were raised against rat spleen glycoproteins depleted of the Ia‐A antigen. An IgG antibody (called MRC OX 17) was obtained and used to purify a molecule which had a similar structure to the Ia‐A antigen and reacted with anti‐Ia alloantibodies. There was no cross‐reaction between the two Ia glycoproteins in assays with mouse monoclonal antibodies, alloantibodies or rabbit antibodies. In one alloantiserum almost all the detectable anti‐Ia antibodies reacted with a mixture of the two Ia glycoproteins. The MRC OX 17 antibody did not bind to mouse cells, but rabbit antibodies to the pure rat glycoprotein cross‐reacted and recognized determinants mapping to the mouse I‐E region. In the thymus the rat Ia‐E antigen was on cortical epithelial and medullary reticular cells.

An IgG monoclonal antibody (MRC OX 18) to isotypic determinants of rat histocompatibility RT‐1A antigens was also produced and used to analyze these antigens on thymus cells. The heavily labeled thymocytes were those with characteristics of mature T lymphocytes. Cortical epithelial cells and medullary dendritic‐like cells were also RT‐1A positive.