By using the compartmental analysis approach of pharmacokinetics, found useful in describing the kinetics of a foreign compound in a biological system, a rather simple mathematical model was developed to predict, by computer simulation, steady-state pesticide residues in fish from limited pre-steady-state experimental data. Such steady-state data are needed in order to calculate the tendency of the residue to concentrate in fish--the so-called bioconcentration factor. The model was applied to four different carbon-14 labeled biocides at seven concentrations. Considering the simplicity of the model and the fact that residues were measured by the non-specific technique of radioassay, model predicted values were in good agreement with experimental values.