Introduction

Chlorinated biphenyls were known before the turn of the century and the useful industrial properties of mixtures obtained by chlorination of biphenyl were recognized early. In 1967, Widmark reported mass spectroscopic data as unambiguous proof of the chemical nature of these new contaminants and at about the same time polychlorinated biphenyls's (PCB)'s were found in various parts of the world. In 1966, the discovery of PCB's in environmental samples spurred renewed interest in the toxicity of PCB's and a mass of information on the acute and chronic toxicity of PCB's to a variety of animals has been obtained since then. Most of the studies were carried out using commercial PCB preparations and very little is known about the toxicity of individual chlorobiphenyls. Some highly chlorinated biphenyls may be toxic simply by their presence in the tissues and less chlorinated biphenyls may be toxic by taking part in metabolic processes.