The production of a bacteriocin complex by Lactobacillus acidophilus LF221 at different pH values in MRS broth (5.5, 6, 6.5 and 7) and the mode of action against Lactobacillus helveticus ATCC 15009 were studied. Adsorption of bacteriocins to the producer cells and induction by its own bacteriocins or by sensitive cells was also examined. Bacteriocins were produced during the logarithmic phase of growth. The highest amount of bacteriocin complex (106 AU/ml) was detected in MRS broth at pH 6.5. The bacteriocin complex and biomass production were not directly related. The production of bacteriocin complex was not reduced by low inoculum (4 x 10(-6)) and was not affected by the cells of indicator strain or by its own bacteriocins. No bacteriocins adsorbed to the cells of the producer at any pH tested. The bacteriocin complex at a concentration of 100 AU/ml was bactericidal for growing cells of L.helveticus ATCC 15009 but not bacteriolytic.