Intraperitoneal vaccination of female mice, before mating, with a whole cell, heat-killed (62 degrees C) vaccine of Campylobacter jejuni allowed the mother to confer immunity to her young, challenged orally 4-6 days after birth with the homologous strain. There was no protection against a strain of another serotype. Heating the vaccine to 100 degrees C destroyed its protective properties. A vaccine prepared from an aflagellate variant of the original strain was as protective as the original vaccine against challenge with the flagellated strain. Anti-flagellar serum antibody titres of the dams did not correlate with protection of their young.