The effectiveness of the delivery of amoxicillin to Atlantic salmon, undergoing chemotherapy in natural outbreaks of furunculosis in sea-cages, was investigated by measuring the concentration of the drug in serum samples. Five groups of 50 sera from three outbreaks were collected two hours after oral treatment with doses of 80 or 120 mg/kg bodyweight. Amoxicillin was detected in 82, 82, 92, 100 and 90 per cent of the sera in the five groups (limit of detection 0.16 microgram/ml). Many sera contained less than the minimum inhibitory concentration of amoxicillin for the causative agent Aeromonas salmonicida (0.3 microgram/ml), but a concentration more than double the minimum inhibitory concentration was achieved in 2, 2, 56, 32 and 44 per cent of the samples. There was wide variation in the serum concentrations between individuals in the same population and between populations receiving the same treatment; this variation was associated with population factors, the severity of infection and the accuracy of medicating the feed.