The effluent cause of a noninfectious hyperbilirubinaemia. or jaundice, in wild Don salmon was determined by exposing adult captive North Esk fish to environmentally relevant single and combined exposures of Donside discharges with control river water in situ. Adult North Esk salmon were chosen as test animals because they were physiologically pertinent, of different genetic origin or stock to Don fish, from a 'clean' river and successfully used in previous in situ riverine experiments.Experiments were conducted using a control group with 3 test groups of salmon exposed to individual or sequentially-combined industrial stream and untreated paper mill effluent for a test period of 4 wk. Sequential exposure of industrial stream followed by paper mill effluent produced haematological 'clinical profiles', i.e. a hyperbilirubinaemia in test North Esk salmon that was indistinguishable from wild Don fish. A combination of 2 types of effluent therefore appeared to be responsible for the noninfectious hyperbilirubinaemia reported in adult wild Don salmon during the previous decade. Chemical characterisation of the isolated effluents and the river are discussed with respect to known haemolytic agents from the tox~cological l~terature.