The physiological significance of casein kinase II (CK-II) on the protease (PR) activity of recombinant HIV-1 PR (rPR) was biochemically investigated in vitro. We found that (i) the purified rPR (pll) functions as a phosphate acceptor of CK-II; (ii) the PR activity of rPR is stimulated approximately 2.9-fold after its full phosphorylation by recombinant human CK-II (rhCK-II) in a manner similar to that observed for recombinant HIV-1 reverse transcriptase (rRT); and (iii) this stimulation is completely inhibited by two polyphenol-containing anti-oxidant compounds [quercetin and epigallo-catechin gallate (EGCG)] at 0.1 mu M or a glycyrrhetinic acid derivative (oGA) and catechin at 10 mu M without significant effect on the PR activity of rPR. These results suggest that (i) CK-II may be a host mediator responsible for stimulation of PR and RT in HIV-1-infected cells; and (ii) the selective inhibition of the CK-II-mediated stimulation of HTV-1 PR and RT by potent CK-II inhibitors may be involved in their anti-HIV-1 effects at the cellular level.