Isoimmunization of heifers by a single injection of semen and complete Freund's adjuvant up to 21 days after insemination resulted in a high incidence of early embryo loss. The heifers had a pregnancy rate of 11·8% and an incidence of delayed returns to oestrus of 41·3%. Only one of ten heifers immunized with semen had a fertilized ovum at slaughter after the second to fifth insemination. Pregnancy occurred in eight of ten control heifers immunized with seminal plasma after insemination.
Heifers immunized with homologous conceptus material and adjuvant by repeated intra-uterine and intradermal injections required an average of 3·8 inseminations per pregnancy. The infertility was apparently due to early embryo death. Isoimmunization with seminal plasma and immature testis, and the treatment of semen with antisera to conceptus and immature testis, had no effect on fertility in heifers.