This Test Guideline describes a procedure for characterising the bioconcentration potential of substances in fish, under flow-through conditions (but semi-static regimes are permissible). The test consists of two phases: the exposure (uptake) and post-exposure (depuration) phases. During the uptake phase (28 days normally and 60 days maximum), separate groups of four fishes of one species are exposed to at least two concentrations of the test substance. They are then transferred to a medium free of the test substance for the depuration phase. A depuration phase is always necessary unless uptake of the substance during the uptake phase has been insignificant. In addition to the two test concentrations, a control group of fish is held without the test substance. The concentration of the test substance in/on the fish is followed through both phases of the test. During the test, dissolved oxygen, TOC, pH, total hardness and salinity, and temperature should be measured in vessels. The lipid content should be determined on the same biological material as is used to determine the concentration of the test substance, when feasible. Where possible the bioconcentration factor at apparent steady-state (BCF), expressed as a function of the total wet weight of the fish, and the kinetic bioconcentration factor (BCFK) are calculated. The bioconcentration should be expressed in relation to lipid content in addition to whole body weight.