We have investigated the potential contraceptive effects of immunization against the luteinizing hormone (LH) receptor in male mice at the prepubertal stage. Two N-terminal fragments of the porcine LH receptor encoding amino acids 1-297 and 1-370 were produced in large quantities through the Baculovirus insect cell system. We have immunized three-week-old mice from two Balb/c stocks of differing fecundity with Sf9 insect cells producing the short (1-297) or long (1-370) recombinant LH receptor. A booster injection was performed at six weeks using purified antigens. Ten days later, the immunized male mice were mated over a period of two weeks with adult untreated females. After weaning of the first litters, the same partners were mated once again under the same conditions. There was no decrease in the antiserum titers against the antigens over a two-month period. The circulating testosterone decreased as the anti-LH receptor antibodies increased. The fertility of the treated male mice was reduced up to 75%, depending on the mouse stock, the antigen used and the time separating immunization and mating. The impaired fertility was mostly due to male sterilization (up to 60% of sterile mates). The delay between mating and birth was enhanced by the treatment, reflecting delayed fertility and/or delayed male behaviour acquisition.