The peritoneal cells of mice injected with aclacinomycin (ACM), an oncostatic drug of the anthracyclin family, were found to secrete more interleukin (IL-1), after two successive 24-h periods of in vitro LPS stimulation than those of control mice. This measured IL-1 production is one of the signs of enhanced macrophage activity. The cells of ACM-injected mice also contained more intracellular IL-1 than those of controls. In contrast, macrophages from ACM-injected mice only increased their IL-1 production after the first 24-h incubation with PMA, and not after the second 24-h incubation. The response to ACM was dose- and time-dependent. We have also compared the IL-1 production by macrophages from mice injected with other anthracyclins, at doses equimolar to that of 4 mg/kg ACM and we have observed that adriamycin, 4'-epiadriamycin and aclacinomycin had similar activity, while THP-adriamycin an daunorubicine were slightly more active. Exploitation of this increased IL-1 production by macrophages could be beneficial in the design of tumor treatment protocols.