The microbial degradation of monochloro-, 1,2-dichloro-, 1,4-dichloro-, and 1,2,4-trichlorobenzene in soil slurries was examined with single compounds as well as in mixtures. The indigenous soil populations brought about the degradation of monochlorobenzene when incubated at 27 degrees C in slurries with 29% (w/w) suspended solids. In contrast, the other chlorobenzenes persisted during an incubation period of 1 month. Supplementation with buffer, mineral salts and acetate did not significantly influence the degradation. However, inoculation with Pseudomonas aeruginosa strain RHO1, a monochloro- and 1,4-dichlorobenzene-degrading organism, to a titre of 1 x 10(5) cells/g soil, led to rapid and complete degradation of 0.8 mM growth substrate within 30 h. In addition, the strain was able to degrade 1,2-dichloro- and 1,2,4-trichlorobenzene with stoichiometric release of chloride in the presence of acetate, ethanol, monochloro- or 1,4-dichlorobenzene as growth substrates. In mixtures of chlorobenzenes the co-metabolism of 1,2-dichloro- and 1,2,4-trichlorobenzene occurred until the growth substrates monochloro- and 1,4-dichlorobenzene were degraded. The degradation was faster in the slurries of garden soil containing 8% organic carbon than in soil with the lower content of 2.6%.