Human occupational exposure to polychlorinated biphenyls (PCBs) with varying chlorine content has been reported by several investigators, using analyses of blood or adipose samples or skin wipes to evaluate levels in the body. The intensity of occupational exposure is related to both duration and intensity of exposure. The qualitative nature of occupational exposure, as well as casual environmental exposure, has been shown to consist of less readily metabolized PCB congeners. The pattern of PCB congeners in human tissues, determined by gas chromatography, may or may not be readily ascribed to specific PCB standard mixtures. The average occupational exposure, as depicted in several studies of blood, plasma or serum concentrations, is approximately 10 to 1000 times that observed in nonoccupationally exposed persons. Currently used methods of PCB quantitation and pattern identification vary widely, with no uniformly administered criteria being applied to characterize human PCB exposure.