Mouse placenta has been found to contain an mRNA that encodes a previously unidentified member of the prolactin-growth hormone family. This 1.1-kb mRNA (designated PRP mRNA) was detected as a cDNA clone that hydridized to a cDNA clone of mouse proliferin, a recently described growth-associated placental protein related to prolactin. PRP mRNA levels are highest in the fetal part of the placenta and peak at day 12 of gestation, decreasing gradually until term. The 972-bp sequence of PRP mRNA, determined from two cDNA clones, encodes a protein of 244 amino acid residues that has a hydrophobic leader sequence. The protein encoded by PRP mRNA has significant homology to all of the members of the prolactin family, yet is different from each of them; it also differs from mouse placental lactogen. Nucleotide sequence homology is most extensive between PRP and proliferin mRNAs, particularly at their 5' ends, where they share 92 of the first 97 nucleotides.