Sieved agricultural soil samples were treated with the anti-knock agent tetraethyl lead (Et4Pb), and the resulting effects were analyzed by microcalorimetry. Et4Pb additions resulted in an increase of the heat production rate, provided that oxygen was present and that the soil was not autoclaved. The increased heat production rate was accompanied by degradation of Et4Pb, as verified by speciation analysis (GC-MS) of the remaining Et4Pb and its ionic degradation products (triethyl lead and diethyl lead cations). Conclusive evidence was obtained that these transformations were mediated mainly by microbes. At an initial Et4Pb concentration of 2 g Pb/kg dry weight the biodegradation rate was about 780 mu mol day(-1) kg dry weight(-1), whilst the chemical decomposition was only 50 mu mol day(-1) kg dry weight(-1). A fivefold rise of the initial Et4Pb concentration resulted in a decrease of the biodegradation rate to 600 mu mol day(-1) kg dry weight(-1) and an increase of the chemical decomposition to 200 mu mol day(-1) kg dry weight(-1). The biodegradation rate was not influenced by the addition of glucose, which means that no indication for a co-metabolic attack of Et4Pb was found.