In 1988 and 1989 tissue samples were obtained from the grey seal (Halichoerus grypus) population found in the Dee estuary in the north west of England and from harbour seals (Phoca vitulina) from the populations in the Wash and north east Scotland and analysed for mercury and organochlorine compounds. Adult seals from the Dee estuary were highly contaminated with mercury and polychlorinated biphenyls (PCBs), and one animal from the Dee contained traces of dichlorodiphenyltrichloroethane (DDT), suggesting the recent use of this banned pesticide. The levels of hexachlorobenzene in the livers of two Dee seals exceeded those in the blubber, possibly indicating liver malfunction or recent exposure. The same relationship was found for hexachlorobenzene in three specimens from the Wash and, in one of these animals, the liver was also more highly contaminated than the blubber with dieldrin and PCBs. Levels of contamination were lower in seals from the Wash and even lower in animals sampled in Scotland, where only dichlorophenyldichloroethylene, the metabolite of DDT, was routinely detected. The toxicological significance of the results is discussed, particularly in relation to the mortality observed in the seal epizootic of 1988.