1. Humoral immune responses to allogeneic blood transfusions and semi-allogeneic pregnancies were monitored by the indirect haemagglutination assay in strains of inbred rats.
2. Similar titres and ranges of cross-reactive allo-antibodies were induced by blood transfusions and pregnancy; the maternal alloantibody response to paternal antigens was not increased by an unrelated antigenic stimulus.
3. Immunosuppression with cyclosporin, A was effective in preventing the primary humoral immune response produced by blood transfusion but not in abrogating the established immune response to previous pregnancy.
4. These studies have established an animal model of sensitization that both stimulates, the clinical situation of many potential renal transplant recipients and permits analyses of the different antigenic stimuli involved.