Conditions are reported for efficient protoplast formation and regeneration in four strains of Leuconostoc mesenteroides. Protoplasts were produced from each strain at frequencies greater than 99%, although the rate of their production was variable from strain to strain. Bovine serum albumin and Mg were required for maximal regeneration, while the presence of Ca was inhibitory. Regeneration frequencies of 16% could be obtained with strain ATCC 10830. This frequency was four- to eightfold higher than the frequencies of the other strains examined.