Calretinin, a calcium-binding protein, is primarily expressed in certain subtypes of neurons. It has also been found to be present in mesothelial cells and mesotheliomas but not in many types of carcinomas. Using a polyclonal anti-calretinin antibody, we investigated the expression of calretinin immunohistochemically in nonneoplastic human ovaries and testes and ovarian sex cord-stromal tumors (SCSTs). In ovaries, calretinin was expressed in theca interna cells, hilus cells, and scattered individual stromal cells. Oocytes, granulosa cells, theca externa cells, rete ovarii, and most stromal cells were negative. Expression of calretinin was also seen in the ovarian surface epithelium and in collapsed and flat epithelial inclusion glands (EIGs), but not in round, columnar, and ciliated EIGs. In some glands, a transition from calretinin-positive to calretinin-negative epithelium was observed. In postpubertal testes, calretinin was expressed in Leydig cells, but not in germ cells or most rete testes and Sertoli cells. In ovarian SCSTs, strong calretinin staining was seen in all hilus cell tumors (4/4) and the Leydig cell component of Sertoli-Leydig cell tumors (10/10). The Sertoli cell component showed focal weak positivity in 5/10. Fibrothecomas were completely negative (0/8). In granulosa cell tumors, the tumor cells were either completely negative (8/14) or weakly positive at the periphery of the tumor (6/14) while scattered stromal cell staining was seen in 9/14 cases. The expression of calretinin in normal Leydig cells, theca interna cells, the Leydig cell component of Sertoli-Leydig cell tumors, and hilus cell tumors suggests its functional relationship with androgen production. Its pattern of expression in ovarian SCSTs is useful in the differential diagnosis of these tumors. The presence of a transition from calretinin-positive, flat, nonciliated epithelium to calretinin-negative, columnar, ciliated epithelium in the same glands provides strong evidence for mullerian metaplasia.