The effects of plant-microbe-soil interactions on the biotransformation of polychlorinated biphenyls (PCBs) in a rhizosphere soil were investigated. Containers packed with 350 g of a soil contaminated with Aroclor 1242, 1248, 1254 and 1260, were planted with alfalfa (Medicago sativa L.) and augmented with its symbiotic N-2-fixing host rhizobium (Sinorhizobium meliloti, strain A-025). The four treatment setups comprised a factorial combination of the presence/absence of alfalfa with the non-inoculation/inoculation of soil with S. meliloti in a completely randomized design with two replicates, Up to 44 days after planting, when the alfalfa was not fully developed, alfalfa and S. meliloti together were the most effective in PCB transformation/depletion, whereas alfalfa only was the least effective. However, by the last day of the experimental period (Day 270). when alfalfa growth was robust and full, alfalfa alone was the most effective, whereas S. meliloti alone was the least. In rhizobium-inoculated soil. soil hardness increased, soil moisture contents decreased, and both plant growth and yield were lowered, compared to non-inoculated soil. The depletion, loss or change in PCB levels may be attributed to either direct or indirect biotransformation, biotranslocation and adsorption of PCBs due to the presence of alfalfa and/or rhizobial inoculation. Either possibility underscores the possibility of using plant-rhizobacterial associations to phytoremediate soils contaminated with PCBs. (C) 2002 Elsevier Science Ltd. All rights reserved.