Reductive dechlorination is an advantageous process to microorganisms under anaerobic conditions because it is an electron sink, thereby allowing reoxidation of metabolic intermediates. In some organisms this has been demonstrated to support growth. Many chlorinated compounds have now been shown to be reductively dechlorinated under anaerobic conditions, including many of the congeners in commercial PCB mixtures. Anaerobic microbial communities in sediments dechlorinate Aroclor at rates of 3 micrograms Cl/g sediment x week. PCB dechlorination occurs at 12 degrees C, a temperature relevant for remediation at temperate sites, and at concentrations of 100 to 1000 ppm. The positions dechlorinated are usually meta > para > ortho. The biphenyl rings, and the mono-ortho- and diorthochlorobiphenyls were not degraded after a one year incubation. Hence subsequent aerobic treatment may be necessary to meet regulatory standards. Reductive dechlorination of Aroclors does reduce their dioxin-like toxicity as measured by bioassay and by analysis of the co-planar congeners. The most important limitation to using PCB dechlorination as a remediation technology is the slower than desired dechlorination rates and no means yet discovered to substantially enhance these rates. Long term enrichments using PCBs as the only electron acceptor resulted in an initial enhancement in dechlorination rate. This rate was sustained but did not increase in serial transfers. Bioremediation of soil contaminated with Aroclor 1254 from a transformer spill was dechlorinated by greater than 50% following mixing of the soil with dechlorinating organisms and river sediment.(ABSTRACT TRUNCATED AT 250 WORDS)