The kinetics of anti-paternal alloantibody production in syngeneically and allogeneically mated mice previously immunized with allogenic spleen cells has been investigated. In allogeneically mated females, total and cytotoxic alloantibody levels increased during the final 3 days of pregnancy to reach maximal levels around 1 week after parturition. The properties of this alloantibody were indistinguishable from those of alloantibody induced by conventional immunization procedures. In syngeneically mated females no increase was observed. Total hysterectomy demonstrated that the immunogenicity of the conceptus required to produce a secondary humoral immune response was initiated in the majority of females between days 9 and 10 of pregnancy, and that the antigenic exposure by day 10 was sufficient to induce a maximal secondary response. These observations were consistent with the time of appearance of serologically demonstrable H-2 antigens on embryonic cells during pregnancy. These phenomena occurred in 'responder' and 'non-responder' female strains (referring to the ability of allogeneically mated females to produce anti-paternal alloantibody during normal pregnancy) demonstrating that they do not differ in their ability to produce a secondary immune response to alloantigens presented via the intrauterine route. The findings are discussed in relation to the nature of the immunogenic stimulus from the concepts in this system and during pregnancy.