The mRNA for preuteroglobin, a precursor of the hormonally induced protein uteroglobin, has been partially purified from the endometrium of progesterone treated rabbits. The purification procedure starts with total endometrial polysomes and involves treatment with proteinase K and dodecylsulfate, chromatography on oligo(dT)-cellulose, sucrose density gradient centrifugation, electrophoresis in polyacrylamide gels containing dodecylsulfate, and a second absorption to oligo(dT)-cellulose. The final mRNA preparation codes exclusively for preuteroglobin in a wheat germ cell-free system and migrates as a single band in polyacrylamide gels containing 99% formamide. The average length of the poly(A) segment is 60 nucleotides and the translation of the preuteroglobin mRNA is inhibited by m7G(5')ppp(5')A, indicating that it contains a "tcapped" 5'-terminus. Comparison with known standards yields a molecular weight of 200,000 (600 nucleotides) for preuteroglobin mRNA, approximately twice as many nucleotides as required for encoding the 90 aminoacids of its cell-free product.