Extract Ribosomal and cell sap fractions were prepared from first and third trimester human placentas. High endogenous activity was observed with ribosomes prepared from first trimester or third trimester placenta when incubated either in the presence of cell sap derived from first trimester placenta tissue or from Krebs II ascites tumor cells. The homologous placental system is capable of translating poly(U) and globin mRNA, although the latter is translated at about 10% of the efficiency as observed in the homologous ascites tumor system. The placental cell sap may be deficient in a factor necessary for the translation of globin mRNA. The preparation of active ribosomes from first and third trimester polysomes shows that it will be possible to study the endogenous synthesis of placental proteins throughout pregna. These results indicate the feasibility of studying the cell-free synthesis of placental peptide hormones. Human chorionic gonadotropin and human placental lactogen reach peak levels of synthesis at different gestational periods and thus the relation between the synthesis of the hormones and their secretion can be elucidated.