BACKGROUND: Reductive dehalogenation by anaerobic bacteria is a promising method of detoxifying polychlorinated biphenyls (PCBs) in aquatic sediments. PCB-contaminated sediment from Strazsky Sewer river (Slovakia) was used as an inoculum to develop stable sediment-free enrichments with dechlorination activity directed against the commercial PCB mixture Aroclor 1260. RESULTS: Characterization of the PCBs at the site revealed that 45 mol% of the PCBs were dichlorobiphenyls and 62% of the chlorines were in the ortho-position. Both findings are characteristic of extensive dechlorination in situ. Sediment-free enrichments with stable dechlorinating activity against Aroclor 1260 were developed by a series of transfers. These enrichments targeted hexa- through nonachlorobiphenyls and dechlorinated them to tri- through hexachlorobiphenyls by removal of flanked meta- and para- chlorines when pyruvate or lactate was added as a carbon source. The known haloprimers: 4,4'-dibromobiphenyl and 2,6-dibromobiphenyl did not influence the rate or pattern of dechlorination. The chlorophenyl rings targeted were 2,3,4,5,6- (23456-), 2346-, 2345-, 234- and 245-. CONCLUSION: The PCB dechlorination carried out by these enrichments is novel and does not correspond to any of the previously identified microbial PCB dechlorination processes or their combination. Natural attenuation processes may be under way at the Strazsky Sewer river site. Copyright (c) 2012 Society of Chemical Industry