An antibacterial effect of myeloperoxidase, a halide, such as iodide, bromide, or chloride ion, and H(2)O(2) on Escherichia coli or Lactobacillus acidophilus is described. When L. acidophilus was employed, the addition of H(2)O(2) was not required; however, the protective effect of catalase suggested that, in this instance, H(2)O(2) was generated by the organisms. The antibacterial effect was largely prevented by preheating the myeloperoxidase at 80 C or greater for 10 min or by the addition of a number of inhibitors; it was most active at the most acid pH employed (5.0). Lactoperoxidase was considerably less effective than was myeloperoxidase when chloride was the halide employed. Myeloperoxidase, at high concentrations, exerted an antibacterial effect on L. acidophilus in the absence of added halide, which also was temperature- and catalase-sensitive. Peroxidase was extracted from intact guinea pig leukocytes by weak acid, and the extract with peroxidase activity had antibacterial properties which were similar, in many respects, to those of the purified preparation of myeloperoxidase. Under appropriate conditions, the antibacterial effect was increased by halides and by H(2)O(2) and was decreased by catalase, as well as by cyanide, azide, Tapazole, and thiosulfate. This suggests that, under the conditions employed, the antibacterial properties of a weak acid extract of guinea pig leukocytes is due, in part, to its peroxidase content, particularly if a halide is present in the reaction mixture. A heat-stable antibacterial agent or agents also appear to be present in the extract.