The biochemical properties of ovine placental lactogen (oPL) have been previously determined following purification, which has yielded various results. To clarify the properties of oPL prior to purification, oPL was examined in solubilized fetal cotyledonary tissue (d 100 of gestation) or conditioned culture medium by electrophoretic, immunoblotting and immunoprecipitation techniques. In cotyledonary tissue or conditioned culture medium, oPL has an apparent molecular weight (Mr) of 22,000 with an isoelectric point (pI) of 9.2. Incorporation of [3H]-glucosamine or [3H]-mannose into immunoreactive oPL could not be detected, nor did the presence of tunicamycin in explant culture medium alter the apparent Mr of oPL. In vitro translation of d 100 fetal cotyledonary mRNA, followed by immunoprecipitation, provided evidence that pre-oPL has an apparent Mr of 25,000. The size of oPL mRNA was determined to be approximately 1,350 base pairs by Northern hybridization procedures using an oligonucleotide probe which was generated from oPL amino acid sequence data. These experiments suggest that the only intracellular processing oPL undergoes is removal of a amino-terminal signal sequence. We conclude that oPL is synthesized and secreted as a single nonglycosylated-basic protein, at a time during gestation when circulating oPL is elevated.