An extracellular alpha-amylase from Lactococcus lactis IBB500 was purified and characterized. The optimum conditions for the enzyme activity were pH 4.5, temperature of 35 degrees C, enzyme molecular mass of 121 kDa. The genome analysis and a plasmid curing experiment indicated that amy+ genes were located in a plasmid of 30 kb. An analysis of phylogenetic relationships strongly supported a hypothesis of horizontal gene transfer. A strong homology was found for the peptides with the sequence of alpha-amylases from Ralstonia pikettii and Ralstonia solanacearum. The protein of alpha-amylase activity purified in this study is the first one described for the Lactococcus lactis species, and this paper is the first report on Lactococcus lactis strain as a microorganism belonging to amylolytic lactic acid bacteria (ALAB).