A portion of the oxytetracycline administered to fish in marine fish farms is deposited on the sediment under the fish cages. Oxytetracycline is believed to leave these sediment via re-entry into the water column. A simple modelling approach has been developed to quantify the kinetics of this process. The model was applied to data generated in two previously published field studies of the fate of oxytetracycline following its therapeutic use on fish farms. In applying the model, a number of assumptions were made as to the value to be ascribed to parameters for which empirical data were not available. In each case the values set for these parameters were such as to maximise the predicted water column concentrations resulting from sediment out-washing. At a farm where the maximum concentration of oxytetracycline detected in the sediments was 285 mu g g(-1), the maximum water concentration predicted to occur as a result of sediment out-washing was 0.11 mu g g(-1). At a second farm where the maximum sediment concentrations were 10.9 mu g g(-1) the maximum predicted water concentration was 0.016 mu g g(-1). In the presence of the divalent cations Mg++ and Ca++ in sea water it is unlikely that these concentrations are of any biological significance.