Concentrations of 12 selected polychlorinated biphenyl (PCB) congeners and isomers in human serum and adipose tissue after environmental and short- and long-term occupational exposures were determined. The individual congener concentrations in adipose tissue varied between less than 0.01 and 420 micrograms/kg (tri-, tetra-, penta-, hexa-, and heptachlorobiphenyls); the dominating isomers were 2,2',4,4',5,5'- and 2,2',3,4,4',5'-hexaCB, which represented approximately 90% of all PCBs in adipose tissue. Those isomers were also the most abundant in serum specimens after environmental exposure, their contribution together with the 2,2',3,4,4',5',6-heptaCB was approximately 90%. After short-term as well as long-term occupational exposure, the most remarkable changes were observed in the concentrations of the tri- and tetrachlorobiphenyls. In long-term occupational exposure 2,4,4'-triCB (mean 3.15 micrograms/liter) and 2,4,4',5-tetraCB (mean, 9.4 micrograms/liter) showed the highest concentrations, whereas after short-term exposure 2,2',5-tri- (mean 2.04 micrograms/liter) and 2,3',4,4'-tetraCB (mean 1.5 micrograms/liter) were most abundant. In people with only environmental exposure, the concentrations in adipose tissue of some PCB isomers were interrelated. The concentrations of the isomers 2,4,4'5-tetraCB (IUPAC 74), 2,2',4,4',5,5'-hexaCB (IUPAC 153), 2,2',3,4,4'5'-hexaCB (IUPAC 138), 2,3,3',4,4',5-hexaCB (IUPAC 156), 2,2',3,4,4',5',6-heptaCB (IUPAC 183), and 2,2',3,3',4,4',5-heptaCB (IUPAC 171) showed close correlation, the coefficients varied from 0.42 to 0.98. The correlation between the concentrations of individual PCB isomers in adipose tissue was highest for 2,4-substituted highly chlorinated isomers, i.e., isomers with longest half-times in the body.