ABSTRACT:Nya: NYLAR mice were immunized to murine alpha‐fetoprotein (AFP) by active immunization with rat AFP in Freund's adjuvant emulsion before mating or by passive immunization with a high or low dose of whole rabbit antimouse AFP serum or rabbit anti‐AFP IgG at 8–20 days of gestation. In the passively immunized group, anti‐AFP serum or purified anti‐AFP IgG administered at the end of the second week of gestation produced abortion after 24 hours of 41 and 48% of fetuses, respectively. Although abortion did not occur in the low‐dose group, the anti‐AFP serum produced fetal death in 32%, as did the anti‐AFP IgG in 26%, in 72 hours. In the actively immunized group rat AFP produced developmental arrest, but not abortion, in mothers bearing autologous antibodies to mouse AFP. Histopathologic analysis revealed that fetal death resulted from separation of fetal and maternal tissues of the placenta due to subplacental hemorrhages. Immunofluorescent localization of the rabbit IgG implicated both the chorioallontois and yolk sac placenta as