The cytocidal activity of human immune interferon (IFN-γ) in combination with the synthetic double-stranded RNA, poly(I)·poly(C), was investigated in human colon carcinoma cell line HT-29. Three days of treatment with IFN-γ (10 to 25 units/ml) resulted in 30 to 40% reduction in colony formation, whereas poly(I)·poly(C) (25 to 100 µg/ml) reduced cell viability by 10 to 20% of control. The lethal effect of the combination of IFN-γ and poly(I)·poly(C) was synergistic wherein 70 to 90% reduction in colony formation was observed. Measurements of DNA, RNA, and protein synthesis after IFN-γ and poly(I)·poly(C) treatment showed a dose-dependent reduction in all three parameters. Recombinant IFN-γ in combination with poly(I)·poly(C) exhibited a similar effect. Studies evaluating the molecular mechanism of IFN-γ and poly(I)·poly(C) toxicity indicate a lack of involvement of the double-stranded RNA-dependent (2′,5′)oligoadenylate-RNase L and protein kinase pathways; however, the effect appears to be related to the inhibition of ribosomal RNA transcription in this cell line.