Immunological character of alpha fetoprotein (AFP) was investigated in respect to immunological relationship of human and animal AFP; responses to the injection of homologous AFP; responses to injection of human AFP; and effects of maternal antibody against AFP to fetuses. Chickens were immunized with human AFP and no reaction was observed between rabbit or horse antiserum (immunologically cross-reactive) to human AFP and newborn chicken serum. Chicken antibody concentrate was prepared from pooled sera of chicks immunized with human AFP and the antibody concentrate did not react with sera of adult human, dog, cat, pig, rabbit, horse, sheep, goat, cow, rat, and mouse, but did form specific immune precipitates with human AFP and newborn sera of these 10 animal species. Next, rabbits, dogs, and rats received injections of sera of newborn homologous species, but no antibody to AFP was demonstrated by Ouchterlony test, indicating immunological unresponsiveness of adult animals to homologous AFP. Injection of human AFP provoked antibodies to homologous AFP in rabbits, rats, and horses; the dog did not react with human AFP. Finally, antibody to homologous AFP was raised in female rabbits by injecting human AFP, and rabbits were bred resulting in pregnancy to examine the effects of the maternal antibody to fetuses. Sera of newborns and mothers were tested for antihuman, antirabbit, and rabbit AFP. Newborn rabbits gave positive tests for antibody to human AFP, suggesting the transfer of maternal antibody to fetuses during pregnancy.