Abstract Interieukin-1α (IL-1α) exerts antiproliferative effects on a human ovarian carcinoma cell line, NIH : OVCAR-3, which is resistant to clinically relevant concentrations of doxorubicin (DOX) and other chemotherapeutic agents. This action of IL-1α depends on the presence of type I (80 kDa) receptors, although no quantitative relationship has been established between receptor occupancy and inhibition of cell growth. When NIH : OVCAR-3 cells were exposed to IL-1α and DOX in combination, a mutual potentiation of the antiproliferative effects of the two agents was observed. This synergistic effect was not due to IL-1 receptor expression up-regulation by DOX, and receptor-dependent intemalization of the cytokine was also unaffected. The involvement of IL-1 receptors is supported by the observation that synergism between the two agents was diminished (but not abolished) in the presence of a specific IL-1 receptor antagonist at concentrations blocking more than 75% of IL-1α binding. DOX was found to significantly increase IL-1α accumulation by NIH : OVCAR-3 cells after long-term (48 hr) exposure to the cytokine at 37°, which might be due to increased nonspecific fluid phase uptake or to interference with cytokine degradation and/or release processes. The potent synergy of IL-1α and DOX against ovarian carcinoma cells in vitro suggests that this drug combination may be effective against this disease in the clinic.