We compared the cross-reactive pattern of alloantibodies from conventional anti-DA alloimmunizations with anti-RT1Aa alloantibodies brought about by secondary and primary pregnancy-induced alloimmunizations caused by pregnancies by DA males. As expected, conventional alloimmunization produces alloantibodies that are cross-reactive; secondary pregnancy-induced anti-RT1Aa alloantibodies induced by pregnancies by DA males in females previously immunized against DA were as cross-reactive as conventionally raised antibodies. But primary pregnancy-induced anti-RI1Aa alloantibodies brought about by DA pregnancies in unimmunized females were not as cross-reactive as alloantibodies from conventional or secondary pregnancy-induced alloimmunizations. The cross-reactive pattern of the alloantibodies from primary pregnancy-induced alloimmunizations was distinctive for each of the three responding strains, but the cross-reactive pattern observed for a particular strain was not uniformly seen in all the responding females of that strain. One such pattern between two congenic strains defines an Ir gene between two RT1 phenotypes that are known to be high responders to RT1Aa.