In this study we have evaluated the ability of interferons (IFNs) alpha, beta, gamma and Tumor Necrosis Factor (TNF) alpha to modulate the expression of the Major Histocompatibility Complex (MHC) antigens in human osteosarcoma cells. The osteosarcoma cell lines Saos-2 and U-2 OS, treated in vitro with IFNs and TNF alpha, showed an increased expression of class I HLA antigens. However, only IFN gamma and, to a lower extent, IFN beta induced the expression of class II HLA antigens. These effects were dose and time-dependent. Simultaneous treatment with IFN gamma and IFN beta or TNF alpha, which by itself was unable to induce the expression of class II HLA antigens, produced different effects on the two osteosarcoma cell lines: in Saos-2 IFN beta and TNF alpha amplified the effects obtained with IFN gamma alone; in U-2 OS, TNF alpha increased the expression induced by IFN gamma on class II HLA antigens, whereas IFN beta antagonized the effects of IFN gamma. IFN alpha did not influence the induction of class II HLA antigens by IFN gamma in the two osteosarcoma cell lines. IFNs have been introduced in some clinical protocols for the treatment of osteosarcoma, based on their antiproliferative activity. Our findings may contribute to a better knowledge of the effects of IFNs and TNF alpha in osteosarcoma by showing the existence of more complex interactions.