The mechanisms and kinetics of the immunosuppressive effects of alternate-day prednisone were investigated in a group of patients with a variety of inflammatory diseases receiving a range of alternate-day prednisone doses from 5 to 120 mg. Total circulating lymphocyte and monocyte counts, as well as proportions of lymphocyte subpopulations defined both by surface markers and by in vitro functional capacities, were studied. At 8 a. m. of the day on prednisone, just before drug administration, lymphocyte and monocyte counts, proportions of lymphocyte subpopulations, as well as in vitro lymphocyte blastogenic responses to various mitogenic and antigenic stimuli were normal. 4 h after the administration of prednisone, there was a profound lymphocytopenia and monocytopenia, with a differential depletion of thymus-derived lymphocytes as well as various functionally defined lymphocyte subpopulations. Lymphocyte kinetic studies using a radioactive chromium-labeled autologous lymphocytes showed that the lymphocytopenia was due predominantly to a transient depletion of the recirculating portion of the intravascular lymphocytepool. All these parameters returned to normal by 8 a.m. of the following day (off prednisone) and remained normal throughout the day. This very transient lymphocytopenia and monocytopenia after prednisone, with normal cell numbers, proportions, and functions throughout the remainder of the 2-day cycle, was associated with suppression of disease activity, yet did not affect cutaneous delayed hypersensitivity in these patients nor increase the likelihood of infectious complications. This drug-associated cyclic and transient monocytopenia and selective lymphocytopenia is best explained by a redistribution of recirculating lymphocytes to other body compartments, particularly the bone marrow.