Rowson, Lamming and Fry1 and Lamming and Rowson2 have reported that purulent infections rapidly become established in the uterus of the cow following introduction into the uterus of infected bull semen or cultures of C.pyogenes during the luteal phase of the œstrous cycle, but infections introduced into the uterus of cows in œstrus fail to become established. Using ovariectomized cows, the resistance and susceptibility of the bovine uterus to infection was shown to be due to the action of œstrogens and progesterone respectively. The possibilities suggested to account for the variation in resistance to infection include: (a) an increase in leucocyte activity during œstrus, (b) an increase in antibody activity during œstrus, (c) the production in the uterus during the luteal phase of an environment suitable for rapid bacterial development. The present work is concerned with the effects of the uterine environment.