Interleukin-1 (IL-1) has been shown to ameliorate the hematopoietic toxicities of antitumor chemotherapeutic agents in both mice and humans. However, IL-1 toxicity in humans is considerable and is similar to the systemic inflammatory toxicities induced by IL-3, IL-6, and other cytokines with pleiotropic biologic activities, eg, fever, nausea, malaise, and hypotension. We hypothesized that corticosteroids may reduce IL-1 toxicity without reducing IL-1 hematopoietic effects in vivo. C3H/HeJ mice (female, 6 weeks) were treated for 7 days subcutaneously with cortisone acetate (CA), (0.1, 0.25, or 0.5 mg/d/mouse), intraperitoneally with IL-1 (1 or 2 micrograms/d/mouse), or both. As expected, IL-1 increased white blood cell counts, splenic granulocyte-macrophage colony-forming units, and spleen cell number, and protected mice from lethal doses of carboplatin (200 mg/kg; Paraplatin, Bristol Laboratories, Evansville, IN) administered the day after completion of the 7 days of IL-1 administration. CA did not significantly block the hematopoietic effects of IL-1 or the ability of IL-1 to protect mice from the hematopoietic toxicity of carboplatin. IL- 1 administered to mice at 8 micrograms/d/mouse for 5 days induced decreased activity, roughening of hair, diarrhea, pancytopenia, multiple metabolic abnormalities, and death in 60% of mice. IL-1 at the therapeutic doses (0.5 to 2 micrograms/d) was not toxic. CA in a dose- dependent manner blocked all of the above mentioned toxicities when administered 24 hours and 30 minutes before each IL-1 injection. CA also decreased IL-1-induced increase in plasma tumor necrosis factor levels at the time point examined.