To identify androgen target cells in the human anterior cruciate ligament, immunohistochemical localization of the androgen receptor was performed in 31 specimens of the ligament. All of the specimens were obtained at surgery. Seventeen specimens were from women, and 14 were from men: the average age of the patients was 45.2 years (range: 18-78 years). An immunoperoxidase method using monoclonal antibodies to the androgen receptor was employed to identify androgen target cells in the ligament. Consistent staining of the androgen receptor was demonstrable in six specimens obtained from young men 18-24 years old, and equivocal positive staining was seen in two other specimens from young men. No receptors were demonstrated by this method in any of the specimens from women or older men. Androgen receptors were localized to synoviocytes in the synovium and under the synovial lining, fibroblasts in the ligament stroma, and cells lining the blood-vessel walls of the anterior cruciate ligament. The demonstration of androgen receptors in the cells of the anterior cruciate ligament strongly suggests that male sex hormones may have an effect on the structure and composition of this ligament in young men.