Sequential protein production and cell disruption was studied using. E. coli double-lysogen, P90c/(lambda LHL1,phi 434). The induction of the first prophage lambda HL1 to produce beta-galactosidase by the temperature increase was followed by the induction of the second prophage phi 434 by the W-irradiation to lyze the cell. The optimum operating conditions for the UV-irradiation were sought to yield the maximum cell lysis.