An investigation is described concerning the effects of two micro-organism cultures on single compounds and multi-component mixtures of polychlorinated biphenyls (PCBs). Several low chlorine PCB compounds can be degraded easily with these cultures and, in favourable circumstances, some compounds containing up to six chlorine atoms per molecule can be degraded. In certain multi-component commercial mixtures some PCB compounds are degraded more readily than if present singly. The significance of this is discussed, particularly the fact that work on single compounds does not enable predictions to be made about the behaviour of commercial mixtures in the enviroment. Some tentative explanations are suggested for the behavious of the mixtures exposed to these cultures.