Several studies indicate that cell-mediated immune responses, i.e., macrophage (MΦ) cytokine release capacities, myosin heavy chain (MHC) class II (Ia) expression, etc., are suppressed after trauma-hemorrhage in male mice. Testosterone has been shown to be responsible for the depression of MΦ cytokine responses in males after trauma-hemorrhage. Antigen presentation via MHC class II plays a key role in initiating and maintaining cell-mediated and humoral immune responses. It remains unknown, however, whether testosterone has any effect on MHC class II after trauma-hemorrhage. To study this, male C3H/HeN mice were castrated or sham castrated 2 wk before trauma (midline laparotomy) and hemorrhage (Hem; blood pressure 35 ± 5 mmHg for 90 min and resuscitation) or sham operation. Four hours thereafter, MHC class II (Ia) expression was measured using flow cytometry. The results indicate that MHC class II (Ia) expression on peritoneal and splenic MΦ was significantly suppressed in male mice after trauma-hemorrhage. Prior castration, however, prevented the depression in MHC class II (Ia) expression on peritoneal and splenic MΦ after trauma-hemorrhage. Castration did not affect MHC class II (Ia) expression in MΦ from sham-castrated mice. Thus testosterone depresses MHC class II (Ia) expression on peritoneal and splenic MΦ after trauma-hemorrhage in males. Because MHC class II is necessary for an adequate immune response, our results suggest that depletion of male sex steroids or blockade of androgen receptors using agents such as flutamide might prevent immunosuppression via maintaining MHC class II (Ia) expression after trauma and severe blood loss. Copyright © 2006 the American Physiological Society.