Hypoxanthine-xanthine oxidase (HX-XO) system is a classical system that can generate superoxide anions. The inductive effect of the HX-XO system for lambda prophage has been investigated in this study. The results showed that the system can induce lambda prophage from lysogenic state to lytic growth. The inductive effect was directly proportional to the concentration of HX and XO and inversely related to the time of preliminary incubation of HX with XO. The cell density of the lysogenic bacteria also greatly affected the inductive effect. The maximal PFU number of 2.9 x 10(4) PFU/ml was recorded at 0.86 mM HX, 1.6 x 10(-2) U/ml XO, and a cell density of 10(8) cells/ml. The inductive effect of the HX-XO system was inhibited in the suspensions by glutathione, superoxide dismutase, and catalase. The results provide evidence that free radicals are the primary factors in the induction of lambda prophage in lysogenic bacteria.