Polychlorinated biphenyl compounds (PCB) have been reported to be in the environment, and the chronic toxicologic properties have not been reported. PCB compounds with 42,54, or 60% chlorine (1242, 1254, and 1260) are being fed to rats and dogs, and rat and chicken reproduction studies are being conducted at 1, 10, and 100 ppm. After 18 mo in rats (interim sacrifice and histology at 3, 6, and 12 mo), compound 1242 produced no effects. Rats fed compounds 1254 or 1260 had increased liver weights at 100 ppm, but not at 1 or 10 ppm. No other effects have been observed. Dogs fed 1242 have shown no adverse effects, while those fed 100 ppm 1254 or 1260 have not gained weight as well as controls. In the rat reproduction study there was decreased survival of pups at 100 ppm of 1242 or 1254, and decreased mating indices with 1242. No adverse effects at any of the 3 levels of 1260 or at 1 or 10 ppm of the other two compounds have been observed. There was anorexia, loss of body weight, decreased thickness of egg shells, and poor hatchability of eggs from chickens fed 10 or 100 ppm of 1242, or 100 ppm of 1254. At 1, 10, and 100 ppm of 1260 and at other levels of 1242 and 1254 there were no effects.