IT has been proposed that p68, a nuclear protein of relative molecular mass 68,000, functions in the regulation of cell growth and division1. A complementary DNA analysis of the protein2 has revealed extensive amino-acid sequence homology to the products of a set of genes recently identified in organisms as diverse as Escherichia coli and man, which include the eukaryotic translation initiation factor elF-4A (refs3-9). The protein products of the new gene family have several motifs in common which are thought to be involved in nucleic acid unwinding10-12. As yet, however, only elF-4A, through its effect on RNA, has been shown to possess unwinding activity13,14. Here we report that purified p68 also exhibits RNA-dependent ATPase activity and functions as an RNA helicase in vitro. The protein was first identified by its specific immunological cross reaction with the simian virus 40 large T antigen1, the transforming protein of a small DNA tumour virus15. Surprisingly, T antigen also has an RNA-unwinding activity16: the homology between the two polypeptides, although confined to only a small region resembling the epitope of the cross-reacting antibody (PAb204) (ref. 2), should therefore be of functional significance. Furthermore, the RNA-unwinding activity may be involved in the growth-regulating functions of both proteins.