Interleukin-1 alpha induced an increase in both the cellular accumulation of cis-diamminedichloroplatinum (II) (cisplatin) and DNA platination and significantly reduced the removal of platinum from DNA of human ovarian (NIH: OVCAR-3) carcinoma cells in culture. The combinations of IL-1 alpha and cisplatin were highly synergistic against these ovarian carcinoma cells and maximum levels of sensitization (15-20-fold) were observed during simultaneous exposure of cisplatin and IL-1 alpha. IL-1 alpha specific receptor antagonist decreased this synergy. These results strongly indicate that IL-1 alpha inhibits DNA repair, and this inhibition of DNA repair may explain, in part, a strong synergistic interaction between IL-1 alpha and cisplatin in NIH: OVCAR-3 cells.