In 1977 and 1985, serum polychlorinated biphenyl (PCB) concentrations were determined for 58 workers in a Bloomington, Indiana, factory that used polychlorinated biphenyls (PCBs) in capacitor manufacture until 1977. Less chlorinated PCBs were quantitated as Aroclor 1242, and more highly chlorinated PCBs were quantitated as Aroclor 1254. The median half-life was 2.6 y for Aroclor 1242 and 4.8 y for Aroclor 1254. However, the half-life varied inversely with the initial serum concentration. This pattern may be a result of continued low-level exposure, variation in the time of exposure, or enzyme induction by PCBs.