The addition of humic acid (HA) to polycyclic aromatic hydrocarbon (PAH) contaminated systems has been shown to enhance, inhibit, or have no effect on the biodegradation of these PAHs. In this study, the surfactant effects of Elliott soil HA (ESHA) at two pH values were tested. At pH 7.0, ESHA did not behave as a surfactant. At pH 11.8, ESHA acted as a surfactant, as displayed by a decrease in surface tension with increasing concentrations of ESHA. The effect of ESHA on pyrene solubility was tested by adding 0 to 800 mu g ESHA/g soil to soil-slurries. Enhancement of pyrene apparent solubility demonstrated a dose- and time-related effect. Broader doses from 0 to 10,080 mu g ESHA/g soil and three higher doses from 3,360 to 10,080 mu g ESHA/g soil were tested for their effects on pyrene mineralization by indigenous soil microorganisms and a novel PAH-degrading Mycobacterium sp. KMS in soil microcosms, respectively. ESHA amendments between 20 and 200 mu g ESHA/g soil were found to consistently increase pyrene mineralization by indigenous microorganisms, while the 10,080 mu g ESHA/g soil produced inhibition and all other doses presented no effects. Pyrene degradation by M. KMS was significantly inhibited by the addition of the highest dose of ESHA.