Adenosine 5′-phosphate (AMP)** specific IgM and IgG antibodies were obtained by immunization of rabbits with an AMP-E. coli conjugate prepared by coupling periodate-oxidized AMP with the cells. The antibodies were purified using an AMP-oyalbumin (AMP-OA)-agarose conjugate as immunoadsorbent. IgM antibody was separated from IgG antibody by gel-filtration with Sephadex G-200. Relatively large amounts of IgM antibody were separated from the early immune sera. Both the IgM and IgG antibodies showed complement fixing reactions with heat-denatured deoxynucleic acid (d-DNA), but markedly' weak reactions with native DNA. The hapten inhibition technique revealed that specificity of these antibodies was directed toward AMP, and in particular, adenine base.