A series of experiments was conducted to examine the effects of chemical pretreatment on biodegradation of 14C-labeled PCB congeners in aqueous systems. Fenton's reagent was used to generate hydroxyl radicals (OH) which were successful in partially oxidizing/transforming otherwise recalcitrant molecules of tetrachlorinated PCB, but had little or no impact on the biodegradation of a monochlorinated congener. Application of Fenton's reagent (1% H2O2, 1 mM FeSO4) followed by inoculation with pure cultures Pseudomonas sp, strain LB 400 and Alcaligenes eutrophus, strain H850 resulted in the removal of approximately 38% of 2-chlorobiphenyl and 51% of 2,2',4,4'-tetrachlorobiphenyl in the form of 14CO2. Comparison of the rate and extent of biodegradation of 2,2',4,4'-tetrachlorobiphenyl after the application of Fenton's reagent with the dynamic and final level of radioactivity in the aqueous phase of experimental system suggests two possible means of microbial utilization of tetrachlorinated PCB congener altered by chemical oxidation: (a) consumption of the partially oxidized chemical dissolved in the aqueous phase, and (b) direct microbial attack on the transformed compound, which may still be adhered to the solid surface.