Apoptosis of viral infected cells appears to be one defense strategy to limit viral infection. Interferon can also confer viral resistance by the induction of the 2-5A system comprised of 2'-5' oligoadenylate synthetase (OAS), and RNase L. Since rRNA is degraded upon activation of RNase L and during apoptosis and since both of these processes serve antiviral functions, we examined the role RNase L may play in cell death. Inhibition of RNase L activity, by transfection with a dominant negative mutant, blocked staurosporine-induced apoptosis of NIH3T3 cells and SV40-transformed BALB/c cells. In addition, K562 cell lines expressing inactive RNase L were more resistant to apoptosis induced by decreased glutathione levels. Hydrogen peroxide-induced death of NIH3T3 cells did not occur by apoptosis and was not dependent upon active RNAse L. Apoptosis regulatory proteins of the Bcl-2 family did not exhibit altered expression levels in the absence of RNase L activity. RNase L is required for certain pathways of cell death and may help mediate viral-induced apoptosis.