The components present in Haemophilus somnus fractions which suppressed Staphylococcus aureus ingestion and/or iodination of protein by bovine polymorphonuclear leukocytes (PMNs) were identified and characterized. Two fractions which suppressed iodination but not ingestion were isolated by heat-extracting a washed bacterial suspension at 60 C or incubating the bacterial suspension at 37 C and were partially purified by ultrafiltration (HElF-60 and HElF-37 respectively). The components present in these fractions were separated by reverse-phase high performance liquid chromatography (HPLC) and identified as ribonucleotides, a ribonucleoside, and purine and pyrimidine bases. It was found that guanine and guanosine 5'-monophosphate (GMP) were the components responsible for most of the suppression in HElF-60 while guanine and adenine were the major inhibitory components in HElF-37;Through purification by HPLC (anion exchange and reverse-phase) and characterization by chemical analyses, the active component present in the fraction which suppressed both iodination and ingestion by PMNs was identified as ribonucleic acid (RNA). A significant quantity of antibodies were removed from antiserum which contained antibodies to RNA by absorption with Formalin-killed H. somnus. This suggested that RNA was present on the surface of H. somnus cells. Other RNA preparations of different molecular species or sources were also suppressive to both iodination and ingestion;In order to determine if cattle can be induced to produce antibodies against guanine, adenine, and GMP, groups of Holstein calves were immunized with GMP-keyhold limpet hemocyanin conjugate (GMP-KLH), guanosine-KLH, or adenosine-KLH. High titers of antibody to GMP, guanosine, or adenosine developed following the administration of two doses of each antigen. Anti-guanosine antibody crossreacted with deoxyguanosine, guanine, GMP and yeast RNA but not with adenosine or adenine. Similarly, anti-adenosine antibody was found to crossreact with deoxyadenosine, adenine, adenosine 5'-monophosphate, and yeast RNA but not with guanosine or guanine. In contrast, anti-GMP antibody crossreacted only with deoxy-GMP.