Among environmental contaminants, polychlorinated biphenyls (PCBs) are similar to DDT in persistence, biological magnification through food chains, chemical inertness, and hydrophobic character. While toxicity to lower vertibrates and invertibrates is less than that of DDT, mammals are much more susceptible to the toxic effects. PCBs have been implicated in egg shell thinning in some predacious birds, failure to reproduce and deaths in mink, and economic losses in both the dairy and poultry industries. A wide variety of pathologic changes have beenobserved––teratogenesis, edema, damage to liver and kidneys, and retarded growth and development of sex characteristics. Growth retardation was temporary in chicks fed < 40 ppm PCBs in a ration and permanently impaired above this concentration. A by-product, tetrachloroparadibenzofuran found in trace quantities in PCBs of European manufacture, is more toxic than PCBs but has similar biological effects. Considerable national research emphasis is being directed toward full resolution of PCB-associated problems.