The interaction of components of the seminal plasma with the uterine luminal fluid of pro-oestrous rats was studied in vivo and in vitro. It was observed that 1 to 2 hr after mating with an intact male the uterine contents were transformed into a viscous gel, whose protein matrix was largely derived from the secretion of the coagulating glands. The bulk of this gel consisted of a high-molecular-weight (17S) protein, which was precipitated at 50 to 70% ammonium sulphate saturation. The gelation process required the presence, in small amount (< 1/10 by weight), of another protein fraction, precipitable at 30 to 50% ammonium sulphate concentrations from the coagulating gland fluid, as well as bicarbonate ions in concentrations normally found in the uterine luminal fluid of pro-oestrous rats. Both protein fractions contain covalently bound carbohydrate. The coagulable protein in the coagulating glands secretion differed in size, amino acid composition, carbohydrate content and in its behaviour towards ion exchangers from the previously described coagulinogen secreted by the seminal vesicles, whose clotting is initiated by the coagulating-gland-derived enzyme `vesiculase'.