The effect of interferon-alpha or beta on platinum analogues [cisplatin (CDDP) and carboplatin] cytotoxicity was studied in four glioblastoma cell lines (U373MG, T98G, A172 and U118Mg). All cell lines were strongly resistant to the cytotoxic effect of CDDP or carboplatin. Although both interferons were not cytotoxic in all cell lines, they were able to significantly increase the cell platinum-sensitivity. Specifically interferon-alpha increased the magnitude of CDDP-induced DNA interstrand crosslinks. Our findings suggest that interferons are able to induce a very strong potentiation of platinum analogues cytotoxicity in drug-resistant human glioma cell lines.