To optimize the conditions for transformation of Lactobacillus casei ATCC 27092 cells with plasmid pHY300PLK, a shuttle Vector for Escherichia coli and Bacillus subtilis, by electroporation, we investigated the effects of the electrical parameters (voltage and resistance), the concentration of plasmid DNA, the cell age and density, the electroporation buffer, and other factors. Under optimal conditions of 2.0 kV, 100 ohm, and 25 mu F, a transformation efficiency as high as 1.4 x 10(7) transformants per mu g of plasmid DNA was obtained, with a survival rate of about 50%. L. casei YIT 9021, one of the PL-1 phage mutants of the ATCC 27092 strain, was also transformed with the same plasmid under optimal conditions. The transformants were confirmed to harbor the same intact plasmid molecules by agarose gel electrophoretic analysis.