We report the effects of PRL on cultured human endometrial cells (epithelial cells and stromal cells) and discuss the role of PRL in implantation. Essentially similar responses to PRL in cell attachment and growth were obtained both in epithelial and stromal cells: a low concentration of PRL (3–30 ng/ml of PRL) enhanced cell attachment and growth. In contrast, the presence of a high concentration of PRL (over 100 ng/ml) resulted in a decrease in cell attachment and growth in normal culture conditions. Morphological analysis of epithelial glandular reformation on basement membrane extracts indicated that PRL had dual effects on endometrial epithelial cells: as a stimulator in low concentrations and an inhibitor in high concentrations. These data suggest that PRL may influence the functions of human endometrial cells in a dual way depending on its level and plays an important role in the implantation process.