Interferons (IFNs) exert antitumor activities, but the molecular mechanism underlying these effects is poorly understood. IFN-induced, double-stranded (ds) RNA-activated protein kinase (p68 kinase) has long been implicated in mediating the antiproliferative effects of IFN. In addition, recent studies suggest that p68 kinase may function as a tumor suppressor gene. In this investigation we showed that expression of p68 kinase in HeLa cells resulted in a rapid cell death characteristic of apoptosis. Rapid cell death was not observed in cells which expressed a mutant form of p68 kinase (lys296-->arg) indicating that cell death observed is the result of p68 kinase expression and activation. Moreover, infection of HeLa cells with the mutant vaccinia virus lacking E3L gene, which encodes a dsRNA binding protein that acts as an inhibitor of p68 kinase, also resulted in apoptosis. Thus, we propose that human p68 kinase functions as a tumor suppressor gene by actively participating in apoptosis.