The occurrence and extension of avian mycobacteriosis in house-sparrows (P. domesticus) and mountain-sparrows (P. montanus) have been followed under different epidemiological conditions. Out of 2.929 totally examined house-sparrow pathological changes of tuberculosis were found in 3 (0.10%) animals and in 14 (0.48%) animals the mycobacteria were isolated. By an examination of 544 mountain-sparrow the mycobacteria in 12 (2.20%) cases and in 1 case were isolated and the mycobacteria were found in the intestinal contents. Out of 11 different locations the mycobacteria were found only in places where a contact between sparrows and poultry contaminated by TBC was possible. In some cases the sparrow could be a source of mycobacteria for cattle as well. Simultaneously, the occurrence of TBC in sparrows out of cattle stables correlated with high incidence of avian tuberculin reactions in cattle stabled there. The sparrow could be here not only the source but also an indicator of mycobacteria environment infection. All isolated strains of mycobacteria were virulent for poultry. In experimental infections there was proven a high sensitivity of both species of sparrows to M. avium. After intramuscular infection of M. avium suspension the histological changes in liver have been observed after 12 days, the microscopic changes after 21 days and miliary TBC after 35 days. Peroral infection by food contaminated by TBC poultry livers caused TBC changes in liver and spleen with clinical symptoms and death after 91 days. After free contact between TBC poultry and healthy sparrows the tuberculous changes of parenchymatous organs in sparrows were found after 180 days. There was no difference in sensitivity between the house-sparrow and mountain-sparrow. The transfer of M. avium from TBC-infected sparrows to pigs and poultry was successful. After 45 days of join contact among TBC sparrows, healthy pigs and poultry in stables and after further stay of pigs and poultry in contaminated surroundings there was found an allergy to tuberculin after 65 or 178 days, respectively. Simultaneously, the TBC liver and spleen in poultry were found. The house-sparrow and mountain-sparrow are very sensitive to the M. avium infections. The occurrence and extension of TBC infections in sparrows depends first of all on the possibility of contacts with TBC-infected poultry. Infected sparrows can be an important source of M. avium for other species of animals.