Rhodococcus opacus strain TSP203, a Gram-positive biphenyl utilizer, can degrade trichloro-, tetrachloro-, and pentachlorobiphenyl congeners in PCB concentrations of 100 mu g ml(-1) Kaneclor 300 (equivalent to Aroclor 1242). The effect of Ca-alginate entrapment on the PCB-degrading competence of strain TSP203 was studied. Initially, the ability of immobilized cells of strain TSP203 and cells of TSP203 in solution to degrade congeners in 20 mu g ml(-1) Kaneclor 300, Kaneclor 400 (equivalent to Aroclor 1248), and Kaneclor 500 (equivalent to Aroclor 1254) was compared. In Kaneclor 300 and Kaneclor 400, PCB degradation by alginate immobilized and cells in solution were almost equivalent while in Kaneclor 500, degradation by cells in solution was slightly higher. The effect of increasing concentrations of Kaneclor 300 on the PCB-degrading ability of alginate-immobilized cells was evaluated. At 50, 100, and 150 mu g ml(-1) Kaneclor 300 an improvement in degradation by immobilized cells compared to cells in solution was observed For degradation of 100 mu g ml(-1) Kaneclor 300, the optimum pH was 8.4 and optimum temperature 30 degrees C The biomass concentration was found to be critical for degradation. At low biomass concentration, trichloro-, tetrachloro-, and pentachlorobiphenyls were poorly degraded. In semicontinuous experiments, the immobilized cells could be used for three successive batch cultures with decreasing degrees of efficiency while cells in solution lysed after the first rise. Although Ca-alginate afforded protection against toxicity of PCBs, we were unable to obtain a stable regeneration of the the biocatalyst probably due to the absence of phosphate in the regeneration medium.