The effectiveness of the alkycycline 4-demethoxy-3'-deamino-3'-aziridinyl-4'-methylsulphonyl-daunorubicin (PNU-159548, ladirubicin), a new drug with high antitumour activity against a broad range of neoplasms, was evaluated by using a panel of 32 human osteosarcoma cell lines, including cell lines resistant to doxorubicin, methotrexate, or cisplatin. PNU-159548 resulted to be highly active in all cell lines. No cross-resistance was found with conventional drugs, being PNU-159548 active also in cells resistant to doxorubicin and with a multidrug resistance phenotype (associated with MDR1 gene/P-glycoprotein overexpression), as well as in cells resistant to methotrexate or to cisplatin. Analysis of drug-drug interactions showed that PNU-159548 could be successfully used in combination with all the most important drugs currently used in OS chemotherapy. In fact, the simultaneous administration of PNU-159548 and doxorubicin, methotrexate, or cisplatin produced mostly additive or synergistic effects. Sequential exposure to PNU-159548 followed immediately by doxorubicin, methotrexate, or cisplatin was the most effective sequence of administration, invariably resulting in additive or synergistic effects in both drug-sensitive and drug-resistant osteosarcoma cell lines. In conclusion, the high in vitro effectiveness, the absence of cross-resistance with doxorubicin, methotrexate, or cisplatin and the possibility to be successfully used in combination with these drugs indicate PNU-159548 as a promising candidate to be considered for planning new therapeutic regimens for osteosarcoma patients, who show a decreased response to conventional chemotherapy.