The effects of various in vitro and in vivo regimens of corticosteroid administration on guinea pig alveolar macrophages were studied. Corticosteroid-induced immunosuppression was assessed by the effect of drug administration on the total numbers and functional capabilities of alveolar macrophages as measured by the PHA-induced and antibody-dependent cellular cytotoxicity assays against sheep red blood cell targets. In vivo administration of either hydrocortisone sodium succinate (100 mg//kg/one dose) or cortisone acetate (100 mg/subcutaneously for 7 days) caused a marked increase in the numbers of alveolar macrophages recovered from the teased lung cell suspensions at 4 and 24 hr, respectively, after the last injection. Both regimens of corticosteroid administration cause similar levels of peripheral blood lymphocytopenia and monocytopenia, 4 and 24 hr, respectively, after the final injection. Neither in vitro hydrocortisone (0, 1, and 10 µg/ml), nor hydrocortisone (100 mg/kg), in vivo had any effect on either the PHA-induced or antibody-dependent cellular cytotoxicity of alveolar macrophages. In marked contrast, cortisone acetate, depo-preparation which gives sustained elevations of plasma cortisol levels similar to those found for a brief period after i.v. injection of hydrocortisone caused a marked decrease in cytotoxic effector function of alveolar macrophage suspensions. In a separate experiment, the suppressed killer cell function of the alveolar macrophages from steroid-treated animals was found not to be related to an intrinsic defect in killing of bound target cells since the defect in killing could be overcome by increasing the density of antibody and PHA on the target cells.