Intranuclear inclusions sometimes are observed in endometrial glands, especially during puerperium, and superficially they resemble those from herpesvirus-infected cells. To study the significance of the inclusions, endometrial and placental tissue from 215 subjects, including 69 pregnancy-associated samples, were examined. With screening by peroxidase-labeled avidin alone, followed by the peroxidase reaction, only pregnancy-related endometrium (in 32 cases) demonstrated reactivity in the intranuclear inclusions. These intranuclear inclusions demonstrated positive immunostaining for biotin using the peroxidase-antiperoxidase method in all but six cases, whereas there was no reaction for Herpes simplex in any of the cases. The endometrial tissues of the 32 cases were obtained from the 16th gestational week to the 37th postpartum day. Although the histogenesis of these intranuclear inclusions has not been clarified, they should not be mistaken for those in endometrial cells infected by herpesvirus because of intranuclear biotin and a false-positive histochemical reaction in the avidin-biotin-peroxidase complex method.