 Animal feed was contaminated with several thousand pounds of polybrominated biphenyl in Michigan in 1973 resulting in high exposures of PBB in the population via milk and other products from contaminated cows.Perinatal exposure of children was estimated by measuring PBB in serum of mothers some years after exposure.Girls that had been exposed to high PBB levels through lactation had an earlier age of menarche and pubic hair development than girls who had less perinatal exposure.