Some hydrophobic chemicals may reach plateau levels in fish only after several months of continuous exposure. Therefore, an accelerated test procedure, based on kinetics, was developed using an isomer of PCB's (polychlorinated biphenyls): 2,2′,4,4′-tetrachlorobiphenyl. The rates of uptake and clearance of 2,2′,4,4′-tetrachlorobiphenyl were determined by analysis of rainbow trout (Salmo gairdneri Richardson). These trout were exposed to 1.6 and 9.0 μg/liter for five days and then transferred to fresh water. A nonlinear regression analysis was used to estimate the rate constants for uptake and clearance, and the bioconcentration factor at steady-state was calculated from the rate constants. For 2,2′,4,4′-tetrachlorobiphenyl, the bioconcentration factor at steady-state was 9550 ± 1610 in trout muscle.