Accidental contamination of the food chain in Michigan in 1973 with polybrominated biphenyls (PBBs) led to the establishment of a registry of exposed individuals in 1976. Seram was collected and analyzed for PBB at the time of enrollment and for targeted studies in the following years. We used the archived PBB data to study the elimination ofPBB and to identify factors associated with elimination. A total of 380 women 2 16 years of age who had an initial PBB level of 2 ppb and at least two serum samples drawn when they were not pregnant were induded in the analysis.The mean initial PBB level was 20.9 ppb (median 4) and mean time between the first and last measurement was 4.2 years (range 0.5-11.1). PBB was assumed to reach equilibrium in the body before substantial amounts were eliminated and before the first serum measurements were taken; therefore, the entire body was modeled as a single compartment for PBB with exponential decay.Subject-specific decay rate estimates were regressed on predictor variables icluding initial age, body mass index (BMI), smoking history, breast-feeding duration, and parity. In women with an initial PBB level < 10 ppb, the median half-life was 12.9 years; in those with > 10 ppb, the medi- (91; there-fore, the Michigan individuals were mainly exposed to virtually unchanged PBB153. In the absence of metabolism, elimination is a slow process. Some PBB is eliminated through feces and hair. In women, PBB can be eliminated through lactation. There is also evidence that some PBB is transferred to offspring during pregnancy (4,10,11). The highest PBB concentrations in the body are found in adipose tissue, but it can also be detected at lower concentrations in serum and other lipid-rich tissues, such as the skin and liver.There are two previous reports of serum PBB half-life in women. Both of these studies were conducted using data from the Michigan PBB cohort (12,13