As part of the proliferative response to serum, mouse 3T3 cells produce a set of growth-related mRNAs identified by hybridization to cloned cDNAs. One of these mRNAs, which is about 1 kilobase long, appears within a few hours after stimulation of resting cells with serum or platelet-derived growth factor and reaches a high level during the transition from the G1 to the S phase of growth. This mRNA is translated in vitro into a protein of approximately 25 kilodaltons. The corresponding cloned cDNA of 791 base pairs has been sequenced; it contains a single open reading frame that encodes a protein of 224 amino acids with extensive sequence homology to mammalian prolactins. The initial 29-amino acid segment of the encoded protein resembles the signal sequences of prehormones. That the growth-related protein is not mouse prolactin is indicated by comparison of its predicted amino acid composition with that of mouse prolactin and by the distinct fragment patterns seen when restricted mouse DNA is probed with the cloned cDNA or rat prolactin cDNA. Therefore, the growth-related protein appears to be a new member of the prolactin-growth hormone family. Because of its relationship to prolactin and growth hormone and its association with cell proliferation, the protein has been called "proliferin."