Rat p70s6k and p85s6k have been expressed in baculovirus recombinants propagated in Sf9 insect cells. Surprisingly, both recombinant isoforms were active without coinfection of other kinases which lie upstream in the signaling pathway. Treatment of either recombinant form with phosphatase 2A leads to immediate inactivation in the absence of phosphatase inhibitors. Further studies show that the same four major Ser/Thr-Pro sites associated with p70s6k activation following mitogenic stimulation in vivo are also the four major sites phosphorylated in both the p70s6k and p85s6k during the infection process. It is proposed that the production of phosphorylated and activated recombinant p70s6k and p85s6k is due to activation of a host cell signaling pathway which is triggered by viral infection. In support of this hypothesis, wild-type virus-, but not mock-infected cells, exhibit the multiple phosphorylation of a ribosomal protein which migrates similar to ribosomal protein S6 on two-dimensional-polyacrylamide gels and extracts from these same cells contain elevated levels of S6 kinase activity.