Nearly homogeneous preparations of stromal cells derived from human proliferative endometrium can be obtained by treating endometrial fragments with collagenase in order to disperse stromal elements, filtering the mixture a 25 microns opening sieve to separate them from gland, and incubating the dispersed cells to culture dishes. Exposure of stromal cell cultures to db-cAMP, 8-Br-cAMP or forskolin in RPMI 1640 medium containing 2% ct-FCS and 0.1 U/ml insulin induces the expression of prolactin (PRL), evident from 1) its secretion to the culture medium, measured by radioimmunoassay and by Western blot analysis, 2) the incorporation of 35S-methionine in a protein precipitated with a PRL antibody and co-migrating with authentic PRL during electrophoresis, and 3) the synthesis of PRL mRNA determined by Northern blot analysis. The cAMP effect on PRL production is enhanced by progestins, which by themselves are weak PRL inducers under similar experimental conditions. As expected from previous findings in our laboratory, showing that addition of PRL to the culture medium induces decidualization of endometrial stromal cells, cAMP derivatives not only induce PRL but also provoke differentiation of the fibroblast-like stromal cells to the decidual phenotype, as evident from morphologic changes and by the expression of products characteristic of decidual cells, e.g. IGFBP-1, desmin, hsp 27 and laminin. These findings suggest a PRL-mediated, progesterone-enhanced decidualization mechanism initiated by physiologic agents increasing cAMP levels in stromal cells.