Complementary DNAs (cDNAs) prepared from messenger RNAs (mRNAs) isolated from endometria of 5 day pregnant rabbits were inserted into the plasmid pBR322. A library of 2400 recombinant plasmid clones was prepared and screened by differential in situ hybridization with cDNAs prepared from mRNAs of rabbits either injected with progesterone or untreated by the hormone. Clones encoding uteroglobin were identified and discarded. Several progesterone-induced and progesterone-repressed clones were identified. One of them corresponded to a relatively frequent mRNA (0.2% of clones in the library) of 2300 nucleotides. The induction of this messenger RNA by progesterone was totally suppressed by the antagonist RU486. This compound displayed a limited agonistic activity when administered alone. A very small increase in mRNA concentration was observed after estradiol administration. The messenger RNA was also found in the liver (where it was constitutively expressed), the ovaries, and the Fallopian tubes of rabbits. A cross-hybridizing messenger RNA was detected in human endometrium during the luteal phase. Sequence analysis showed that the messenger RNA encoded a protein of 370 amino acids with a calculated molecular weight of 40,800. A search in Genbank and National Biomedical Research Foundation data banks showed no identity or marked similarity with previously published DNA or protein sequences.