OBJECTIVE: To investigate the effect of the anti-P Org 31710 on human blastocyst attachment to cultured endometrial epithelial cells. DESIGN: Experimental in vitro study. SETTING: University hospital. PATIENT(S): Eleven fertile endometrial donors. INTERVENTION(S): Timed endometrial biopsy for cell cultures. MAIN OUTCOME MEASURE(S): Blastocyst attachment rate on endometrial cell cultures; secretion of glycodelin and leukemia inhibitory factor into the culture medium measured by RIA and ELISA techniques; and expression of progesterone receptors, interleukin-1 receptor type-1, and integrin subunit beta(3) on endometrial epithelial cells examined by immunohistochemistry. Endometrial pinopodes visualized by scanning electron microscopy. RESULT(S): Eleven of 16 human blastocysts attached to control cultures, whereas none of 10 blastocysts attached when Org 31710 was added to the culture medium (P=.0007). Immunohistochemical studies demonstrated no significant differences between groups. Biochemical analyses displayed a trend toward higher glycodelin secretions and, by scanning electron microscopy, a tendency toward less pinopode formation in the Org 31710 group, but the results did not reach statistical significance. The presence of swollen microvilli, precursors of endometrial pinopodes, was significantly reduced on cultures with Org 31710 (P=.03). CONCLUSION(S): The study presents a model for human blastocyst-endometrial interactions responding to an anti-P drug. The exact mechanism for the anti-attachment properties of Org 31710 on the cultured endometrial cells and the blastocysts needs further evaluations.