Agrobacterium tumefaciens cells treated with EDTA were used as a biocatalyst for the transformation of cellobiose to 3-ketocellobiose. The effects of pH of the reaction medium and temperature on the reaction rate and the selectivity during the transformation by the cells were examined, and it was found that the optimum reaction conditions were pH 6 and 15degreesC. The selectivity was about 0.8 under these conditions. The substrate concentration had no significant effect on the selectivity up to 300 mmol/l.