Between 1997 and 2003, in one herd of breeding pigs with 90 sows and two boars, positive and dubious responses to avian tuberculin were detected in 4 and 72 pigs, respectively. Pigs were examined using the agglutination test for the presence of serum antibodies against corpuscular antigens prepared from various <i>Mycobacterium avium</i> complex (<i>MAC</i>) members: <i>M. a. avium</i> (<i>MAA</i>) of serotype 2, <i>M. a. hominissuis</i> (<i>MAH</i>) of serotype 8 and <i>M. intracellulare</i>

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... i>MI</i>) of serotype 19. Positive skin responses were found in animals with antibodies against <i>MAH</i> (18; 23.7%), <i>MAA</i> (3; 4.0%) and MI (9; 11.8%) antigens. By serological examination of 17 sows with repeated dubious responses for tuberculin skin testing with avian tuberculin, no antibodies against <i>MAA</i> were detected; <i>MAH</i> antibodies and <i>MI</i> antibodies were found in eight and two animals, respectively. By <i>post mortem</i> examination of lymph nodes (ln) and organ samples from all 76 animals with responses to avian tuberculin, no tuberculous/tuberculoid lesions were detected. By culture examination of ln and organs from 13 animals, conditionally pathogenic mycobacteria (CPM) were isolated from only one animal (breeding boar): from mesenteric, pulmonary, hepatic ln and from spleen tissue samples. These isolates were identified as <i>MAH</i> and CPM by the PCR method and biochemically. By investigation of the external environment (205 samples), 33 (16.3%) CPM isolates were obtained: 13 <i>MAH</i>, eight <i>M. fortuitum</i>, one <i>M. nonchromogenicum</i>, one <i>M. abscessus</i>, one <i>M. scrofulaceum</i> and nine unidentified isolates, which were non-<i>MAC</i> according to the PCR examination. Non-specific responses obtained in the intravital tests (skin and serological tests) caused by CPM present in the environment substantially complicated diagnosis of avian tuberculosis. Based on these findings, animal hygiene measures have been adopted since 2002; resulted in a decrease of environmental contamination with CPM and a reduction in the number of animals giving positive responses to avian tuberculin.