The synthesis and secretion of placental protein 12 (PP12) were studied in tissue culture using explants of decidua, amnion, chorion, and placenta from seven full term pregnancies. The total amounts of PP12 in media and tissues were measured by RIA, and new protein synthesis and secretion by decidual explants were demonstrated by the incorporation of [35S]methionine into PP12 after 20 h of incubation with 12.5 microCi/ml [35S]methionine. Cycloheximide was used to study the effect of a protein synthesis inhibitor on the secretion of PP12 by decidua. Significantly more PP12 (P less than 0.001) was released into the medium from decidual explants than from chorion and amnion explants throughout the experimental period of 24 h. When incubated under identical conditions, placental explants released no detectable PP12. In decidual tissues and their culture media, the total amount of PP12 was 127.4% higher after incubation than before incubation (P less than 0.001). No increase was found when chorion and amnion were cultured. The addition of cycloheximide into cultures decreased the total amount of PP12 in the decidua and in its culture medium by more than 50%, indicating that one part of PP12 in decidua was performed and another part was newly synthesized. Decidual explants incorporated [35S]methionine into immunoprecipitable PP12 indicating new PP12 synthesis. In gel filtration, 77% of decidual [35S] PP12 eluted in the same position as purified PP12. In sodium dodecyl sulfate polyacrylamide gel electrophoresis, the migration mobility of [35S]PP12 was identical with that of purified PP12. Our results clearly demonstrate that PP12 is a decidual rather than a placental protein.