The impact of corticosteroids on host defense processes has been studied frequently because of its obvious clinical significance in many surgical patients. A technique that quantifies intraleukocyte iodination was used to measure polymorphonuclear leukocyte phagocytosis in heat inactivated and nonheat inactivated sera in the presence of two different corticosteroids: hydrocortisone sodium phosphate (HSP) and methylprednisolone sodium succinate (MSS). HSP produced a significant reduction in phagocytosis in the cells of healthy subjects when both heat inactivated autologous and isologous serum samples were used. MSS showed no such effect. Conversely, MSS produced a significant reduction in phagocytosis in the cells of healthy subjects when nonheat inactivated autologous serum was used but not when isologous serum was used. These data generally tend to confirm previous studies suggesting that MSS is less deleterious to some host defense processes than HSP and may be the steroid of choice when infection threatens the recipient.