A mixed culture of microorganisms able to utilize 4,6-dinitro-ortho-cresol (DNOC) as the sole source of carbon, nitrogen and energy was isolated from soil contaminated with pesticides and from activated sludge. DNOC was decomposed aerobically in batch cultures as well as in fixed-bed column reactors. Between 65% and 84% of the substrate nitrogen was released as nitrate into the medium, and 61% of the carbon from uniformly C-14-labelled DNOC was recovered as (CO2)-C-14. The mixed microbial culture also decomposed 4-nitrophenol and 2,4-dinitrophenol but not 2,3-dinitrophenol, 2,6-dinitrophenol, 2,4-dinitrotoluene, 2,4-dinitrobenzoic acid or 2-sec-butyl-4,6-dinitrophenol (Dinoseb). Maximal degradation rates for DNOC by the bacterial biofilm immobilized on glass beads in fixed-bed column reactors were 30 mmol day(-1) (1 reactor volume)(-1), leaving an effluent concentration of less than 5 mu g l(-1) DNOC in the outflowing medium. The apparent K-s value of the immobilized mixed culture for DNOC was 17 mu M. Degradation was inhibited at DNOC concentrations above 30 mu M and it ceased at 340 mu M, possibly because of the uncoupling action of the nitroaromatic compound on the cellular energy-transducing mechanism.