An improved method for the electrotransformation of Lactobacillus plantarum CECT 220 (ATCC 8014) with plasmid DNA isolated from Escherichia coli is described. The two main modifications with respect to existing methods are: (i) isolation of plasmid DNA from E. coli JM110 grown in minimal medium and (ii) in vitro modification of the DNA by cell-free extracts of the host L. plantarum. Optimal electrotransformation was obtained with exponentially growing cells of L. plantarum concentrated to 6x10(9) cfu ml-1, with electric pulses of 13 kV cm-1 in cuvettes with 1 mm inter-electrode distance. We consider that this method constitutes a useful tool for routine manipulation of L. plantarum, and can probably be extended to other lactic acid bacteria.