The existence of a motility-inhibiting substance in human peritoneal fluid (hPF) has been demonstrated. The results of this study indicate that this factor from hPF is a heat-resistant, volatile molecule with a very small molecular mass (e.g., halothane, which is used for general anesthesia in laparoscopic GIFT). This may be the reason for the conflicting results concerning pregnancy rates in gamete intrafallopian transfer.