An apparently unique protein produced in large quantities by the endometrium of the mare which adheres to, or is incorporated into, the acellular capsule that surrounds the equine conceptus in early pregnancy, has been characterized and partially sequenced. It has a molecular mass of approximately 18 kDa on SDS-PAGE gels and is nonglycosylated as assessed by a sensitive carbohydrate detection kit. Comparison of its first 24 amino-terminal amino acids with all entries in the databases failed to show any significant identity with any other protein sequence. Secretion of the protein appears to be progesterone dependent, as its presence in uterine flushings correlates with peripheral serum progesterone profiles during the oestrous cycle and its secretion can be induced in anoestrous mares by administration of a synthetic progestagen. However, in pregnant mares, the protein disappears from the uterus after about day 20 (term = 320-340 days), despite the persistence of high serum concentrations of progesterone, indicating that additional mechanisms control its synthesis and secretion. The strong association of the protein with the glycoprotein capsule that surrounds the equine blastocyst suggests that it may be incorporated into the capsule as the capsule expands from day 11 after ovulation. Alternatively, or additionally, it may be involved in the transport of nutrients or other substances through the capsule, and may therefore play an important role in the maintenance of pregnancy.