"Haemophilus somnus" fractions which inhibited iodination of protein by bovine polymorphonuclear leukocytes were isolated by heat extracting a washed bacterial suspension at 60 degrees C or incubating the bacterial suspension at 37 degrees C and were partially purified by ultrafiltration. The components in each fraction were separated by reverse-phase high-performance liquid chromatography and identified as ribonucleotides, a ribonucleoside, and purine and pyrimidine bases. Most of the compounds were found to be inhibitory to iodination in a dose-dependent manner. When the effect of each component on iodination at the concentrations present in "H. somnus" fractions was determined, it was found that guanine and GMP were the components responsible for most of the suppression in the fraction isolated by heat extraction, whereas guanine and adenine were the major inhibitory components in the fraction isolated by incubation at 37 degrees C.