Osteosarcoma is the most common primary malignant bone tumor. The peak incidence is in adolescence and the prognosis is very poor. Even after amputation and chemotherapy, many patients who suffer from osteosarcoma die of lung metastases within 2 years. This report documents a study of the in vitro antitumor activity of cytokines against three human osteosarcoma cell lines. The cell lines MG-63, SAOS-2, and TE-85 were incubated with TNF-alpha, IL-1, or IFN-gamma alone or in combination. TNF-alpha, IL-1, and IFN-gamma had antiproliferative activity against all three cell lines. TNF-alpha and IFN-gamma were the most effective against SAOS-2; MG-63 cells were the most sensitive to IL-1, and TE-85 cells were resistant to TNF-alpha and IL-1 but sensitive to IFN-gamma. The synergistic antitumor effect of TNF-alpha plus IFN-gamma, IL-1 alpha, or IL-1 beta or of IFN-gamma plus IL-1 alpha or IL-1 beta was higher than that obtained when the cytokines were employed alone.