A breeding colony of C57BL/6J females mated with DBA/2J males was examined to determine the incidence and mechanism of the development in the females of hemagglutinins to paternal antigens. It was demonstrated that pregnancy rather than cohabitation is a necessary condition for the appearance of these antibodies. The hemagglutinin titers for paternal erythrocytes vary in an individual female mouse. Females strongly positive after a given pregnancy may appear as negative after the next and then positive again after another. Hemagglutinin titers were recorded during a single pregnancy in each of four females which were strongly positive immediately prior to pregnancy. The titer “disappeared” during the 3rd week of pregnancy, but reappeared by 1 week postpartum. No evidence of fetal disease or decreased fertility in the female was found as a result of sensitization.