Three hybridomas which secrete antibodies to Mycobacterium avium were obtained by the fusion of p3ul myeloma cells with spleen cells of mice immunized with M. avium culture sonicate. The reactivity of these monoclonal antibodies was determined in 16 species of mycobacteria by an enzyme-linked immunosorbent assay. An antibody, slesignated Avi-3, reacted only with M. avium and not with the reference strains of the other 15 species of mycobacteria tested, including M. intracelulare, M.

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... osis, and M. lepraemurium. Specfficity of the antibody was confirmed by assay, using a specific DNA probe of M. avium complex in 29 M. avium complex isolates. An antigen was purified from M. avium culture sonicate on a monoclonal antibody Avi-3-coupled affinity column. The purified antigen gave a single band (molecular size, about 27 kilodaltons) upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The antigen Avi-3 showed strong skin test activity in guinea pigs preimmunized with heat-killed M. avium but not in those sensitized with heat-killed M. intracellulare or M. bovis BCG. Purffied protein derivative elicited positive skin reactions in all the immunized guinea pigs. When heat-killed M. avium was used as the immunogen, strong lymphoproliferative responses were observed in cultures stimulated with the antigen Avi-3. These results suggest that M. avium-specific antigen Avi-3 may facilitate the diagnosis of mycobacterial infections.