A method combining ultraviolet (UV) irradiation followed by microbial treatment was successfully applied to the efficient and complete degradation of polychlorinated biphenyls (PCBs). By UV irradiation, most PCB congeners in a methanol solution were transformed into lesser chlorinated compounds containing less than three chlorines. The resultant UV-irradiated PCBs were then subjected to microbial degradation by Pseudomonas alcaligenes TK102, resulting in their complete degradation within a week.