To find an efficient biological method to solubilize waste activated sludge (WAS) from the biological wastewater treatment process, several strains of thermophilic bacteria capable of solubilizing WAS were isolated from sewage sludge compost. The culture supernatants of the isolates were able to lyse vegetable bacterial cells and the lytic activity mainly came from the exoenzyme produced by the isolates. The culture supernatants of the different isolates showed different lysis characteristics. The factors affecting bacterial cell lysis were investigated using E. coli as a model bacterium. The E. coli cells were lysed easily at higher temperature (60 degrees C or 70 degrees C) while little lytic activity by the supernatants of isolates was observed at lower temperature (50 degrees C). The level of pH also had great influence on the lysis of E. coli cells. The E. coli cells in the early stationary growth phase were easier to lyse than those in the late stationary growth phase or death phase.