A method is described for determining biodegradation kinetics of both naturally occurring and xenobiotic compounds in surface and subsurface soil samples. The method measures both respiration and uptake into cellular biomass of 14C-labeled substrates. After separation of the cells and the soil particles by centrifugation, the cells were trapped on membrane filters for liquid scintillation counting. Mass balances were easily obtained. The technique was used to measure metabolic activity in soil profiles, including unsaturated and saturated zones. First order rate constants were determined for amino acid metabolism and for m-cresol metabolism. Saturation kinetics were observed for amino acids and m-cresol. m-Cresol values for uptake often exceeded those for respiration by greater than a factor of ten. Saturation was not observed in many horizons. Frequently, respiration obeyed saturation kinetics, whereas uptake was first order. It is concluded that measuring only kinetics of respiration may lead to severe underestimations of biodegradation rates. (Copyright (c) Springer-Verlag New York Inc., 1988.)