Introducton In the early days of its use, little work was done on the toxicology of the PCBs, and this was only in relation to the risks of occupational exposure. As will be shown many more studies were made as soon as it appeared that the extremely stable PCB's became a threat to the environment and its wildlife, and accidents occurred of acute poisoning in man and animals. These studies have been made with material with different contents of chlorine, from different manufacture, and-as we can

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... now say in retrospect-with different and unknown contents of toxic impurities. For this reason the toxicological information of PCB's is difficult to summarize, but since the character of some important impurities has recently been elucidated, it is well to discuss these first in order to be able to consider their contribution to the overall toxicity of the different preparations studied. These studies were started because of the analogy between the effects of the PCBs (1-6) and some toxic effects associated with toxic factors in crude chlorophenols and in "toxic fat". Effects of the latter are liver damage (7) , chloracne (7,8) and edema formation (9).