Pregnant mice, guinea pigs, rats, and rabbits responded to injections of Moraxella bovis strain EPP-63(300) with abortion, death, embryo resorption, and production of small litters. The nature of response appeared to depend primarily on the number of viable cells injected and to some extent on the species of animal and stage of pregnancy. Intraperitoneal injection of mice with 3 X 10(5) viable cells induced 100% abortion and no deaths. Embryo resorption and smaller litters were induced with injection of lower doses of M. bovis. None of the rats used in this study aborted; however, resorption rates were higher in rats than mice. Bacteria-free filtrates prepared from aqueous extracts of blood agar on which M. bovis had been grown induced abortion in mice and guinea pigs, at a rate similar to that caused by viable cells. The lyophilized filtrate could be diluted to produce 100% abortion with no death of injected mice. Abortion-inducing, nonlethal doses of M. bovis and lyophilized filtrates appeared to have no effect on nonpregnant female or male mice.