Seven technical mixtures of polychlorinated biphenyls (PCBs) were quantified with gas-liquid chromatography-microwave plasma detection (g.l.c.-m.p.d.). These mixtures were then used to determine individual peak response factors for g.l.c.-electron capture detection (e.c.d.), which is used for the routine analysis of PCBs. PCB residues in the blubber lipid from harbour seals, found dead on the Dutch coast from 1972 to 1981 were analysed; 175 samples showed a log-normal distribution with a large coefficient of variation, which makes it difficult to detect differences between groups of harbour seals. Regression analysis showed a significant decrease of PCB content with increasing blubber thickness (BT). Relations between PCB content and age of the seal, and between PCB content and the year in which the seal was found are uncertain.