On 15 occasions, during and after a therapeutic treatment of fish in an Atlantic salmon farm, oxytetracycline concentrations were determined in samples of blue mussels collected at two sites in the vicinity of the farm. Oxytetracycline was not detected in any samples of mussels collected 20 m from the cage black at a depth of 1 m. Those mussels sampled from immediately under one cage (Cage 65) at the farm (10-11 m depth) contained a concentration of 10.2 mu g oxytetracycline g(-1) of soft tissue on the last day of treatment. After the end of oxytetracycline administration the concentration detected in these samples declined exponentially (r(2) = 0.94) with a half-life of approximately 2 days.At the time of the treatment the farm was stocked with 144 tonnes of fish and 186 kg of oxytetracycline were administered during a ten day treatment. In muscle samples taken from fish (n = 5) in Cage 65, after 8 days of therapy the mean oxytetracycline concentration was 1.3 +/- 0.9 mu g g(-1). Three days after the end of the therapy the top 2 cm of the sediments beneath Cage 65 contained 9.9 +/- 2.9 mu g g(-1) (n = 5). These tissue and sediment concentrations lie within the ranges of values that have previously been reported for such treatments.It is argued that residues present in filter feeding bivalves that occur as a consequence of the therapeutic use of oxytetracycline in marine fish farms are unlikely to present a significant human health hazard. (C) 1997 Elsevier Science B.V.