The effects of fractions of Brucella abortus strain 2308 on functions of bovine polymorphonuclear neutrophils (PMNs) were examined in vitro. Ingestion of Staphylococcus aureus and reduction of nitroblue tetrazolium dye by bovine PMNs were not inhibited by heatkilled B. abortus. The ability of PMNs to iodinate proteins was significantly inhibited by live or heat-killed B. abortus and supernatant from heat-killed cells but not by washed heat-killed cells. Two inhibitory components isolated from the supernatant by highperformance liquid chromatography were characterized as nucleotide-like substances with molecular weights of <1,000. Inhibition of iodination by these components was concentration dependent. These results indicate that one of the mechanisms by which B. abortus may escape intracellular killing by PMNs is through the production of low-molecularweight components that inhibit the myeloperoxidase-hydrogen peroxide-halide antibacterial system of bovine PMNs.