ABSTRACT: Certain preparations of rabbit γ-globulins (Cohn Fraction II) contain an activity which, if administered intravenously to pregnant mice, causes either complete fetal resorption in virtually 100% of cases when given during the second trimester equivalent or abortions when given later in pregnancy. All mouse strains tested are sensitive to the activity with slight differences in dosages and kinetics. Unlike vaccines raised xenogeneically against rodent placenta, the activity does not cross-react with kidney or result in any maternal pathology; it can be adsorbed on mouse placental and testicular tissue. Within 1 hr of injection, there is massive venous congestion of the basal decidua and placenta associated with fetal death. There is, however, no evidence of an acute inflammatory reaction or of vascular thrombosis. Expression of the activity can be blocked by pretreatment of the animal with heparin; antihistamines, anticholinergics, and anti-inflammatory agents are without effect. The activity is heat-labile and sensitive to reduction-alkylation and periodation. Virtually all treated females can be successfully remated at a later time.