RNase L is one of the key enzymes involved in the molecular mechanisms of interferon (IFN) actions. Upon binding with its activator, 5'-phosphorylated, 2'-5' oligoadenylates (2-5A), RNase L plays an important role in the antiviral and anti-proliferative functions of IFN, and exerts proapoptotic activity independent of IFN. In this study, we have found that RNase L retards proliferation in an IFN-dependent and independent fashion. To directly measure the effect of RNase L on tumour growth in the absence of other IFN-induced proteins, human RNase L cDNA was stably expressed in P-57 cells, an aggressive mouse fibrosarcoma cell line. Three clonal cell lines were isolated in which the overexpression of RNase L was 15-20-fold of the endogenous level. Groups of five nude mice were injected subcutaneously with either the human RNase L overexpressing clones (P-RL) or control cells transfected with an empty vector (P-Vec). Tumour growth by the two cell lines was monitored by measuring tumour volumes. In the P-RL group, tumour formation was significantly delayed and the tumours grew much slower compared to the control group. Morphologically, the P-RL tumour appeared to have more polygonal cells and increased single cell tumour necrosis. Interestingly, P-RL tumours eventually started to grow. Further analysis revealed, however, that these tumours no longer expressed ectopic RNase L. Our findings suggest that RNase L plays a critical role in the inhibition of fibrosarcoma growth in nude mice.