SummaryThe influence of polychlorinated biphenyls (PCBs) on phocine distemper virus (PDV) infections in harbour seals (Phoca vitulina) was studied. Six out of ten seals had been conditioned with a defined mixture of PCB‐congeneres for several weeks. Following exposure to the cell culture‐propagated PDV isolate 2558/Han88 the complete clinical picture of “1988 seal plague” was provoked in all ten seals inoculated. Four out of six PCB‐conditioned seals and two out of four seals not loaded with PCBs succumbed to the infection within three weeks post inoculation. With regard to the clinical course, duration of cell‐associated viremia, PDV‐antigen distribution in tissues of fatally infected seals and the humoral immune response to PDV no differences between PCB‐loaded and unloaded seals were recognized. Evidence was obtained that the pathogenesis of experimental PDV‐infection in harbour seals shares some features with those of canine distemper in terrestrial carnivores. In contrast, however, to experimental distemper infection of gnotobiotic dogs prompt development of high titres of PDV‐specific IgG did not correlate with reco