Corticosteroids cause an enhanced return of granulopoiesis as measured by in vitro growth of granulocytic progenitor cells (CFU-C) in mice treated with cyclophosphamide. After methylprednisolone and cyclophosphamide, a greater than threefold increase in marrow CFU-C was measured on day 4 compared to mice given cyclophosphamide alone (29,700+/-200 vs. 8,400+/-700/humerus). The accelerated return of marrow CFU-C was observed with cyclophosphamide in doses of 200 and 450 mg/kg and methylprednisolone, 2-20 mg/kg, with no significant differences using >5 mg/kg, and was detected when dexamethasone was used in place of methylprednisolone. This effect was accompanied by similarly enhanced splenic granulopoiesis as measured by CFU-C concentration. Levels of colony stimulating activity did not differ in mice given methylprednisolone and cyclophosphamide or cyclophosphamide alone. Corticosteroids appear to enhance the return of CFU-C by altering the proliferative state of granulocytic progenitor cells. CFU-C survival to in vitro (3)HTdR suicide increased from 72+/-4% on day 1 after cyclophosphamide to 90+/-6% in animals given both cyclophosphamide and methylprednisolone. Increased survival after (3)HTdR suicide was also observed when methylprednisolone alone was given. After treatment with cyclophosphamide and methylprednisolone, blood neutrophils increased more rapidly and improved survival to infection with Candida albicans was observed. These studies demonstrate that corticosteroids have a beneficial effect on marrow regeneration after myelotoxic chemotherapy with cyclophosphamide and suggest that they act by altering cell cycle characteristics of granulocyte progenitor cells.