polybrominated biphenyl or PBB, any of a group of organic compounds used as a fire retardant. In 1973 several thousand pounds of PBB were accidentally mixed with livestock feed that was later distributed to farms in W central Michigan. Some 1.5 million chickens, 30,000 cattle, 5,900 swine, and 1,470 sheep that became contaminated with PBB before the mistake was discovered had to be destroyed. Later studies indicated that PBB had spread through the food chain; in one test of a sample of Michigan's residents, 97% of those tested had traces of PBB in fat tissue. Affected cattle suffered loss of appetite and weight loss (often leading to death), decreased milk production, and increased miscarriages. Laboratory studies have linked PBB with liver cancer in rats and with low birth weight, liver damage, and weakened resistance to disease in human beings.