The effect of Mycoplasma pulmonis, strain JB, on the outcome of pregnancy in TO mice was studied. The mice were infected intravenously before or after mating and the fetuses were examined at autopsy just before parturition. An increase in the number of abnormal pregnancies was noted in mice infected about 2 weeks before mating, and there was a significant increase in the number of fetuses which died mid-way through pregnancy. Mycoplasmas were not isolated from any of the fetuses although the

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... anisms reached the joints of the pregnant mice and caused arthritis. It is possible, therefore, that maternal upset was a factor in these abnormal pregnancies. In mice infected at various times after mating, abnormal pregnancies were most frequently seen in those infected 9 days after mating. There was an increase in the number of both mid-and late-stage fetal deaths in these mice and also an increase in the number of late-stage fetal deaths in mice infected 5 days after mating. Mycoplasmas were isolated not only from most ofthe dead fetuses but also from living ones which suggests that in most instances death was probably due to maternal infection and disturbance rather than fetal infection per se. The possibility of modifying this mouse model by establishing a chronic genital tract infection is discussed as a means of investigating the r\l=o^\leof mycoplasmas in human abortion.