Incidence of allergic reactions to avian tuberculin was investigated in 12,549 birds of the domestic fowl in the territory of one district, in 16 villages and in 721 small flocks. The reactions were found in 7.3% of birds and 35.5% flocks were infected. The zero reactions to tuberculin were observed only in one village out of the 16 villages investigated. Experimental infection confirmed high susceptibility of the domestic fowl to M. avium. After intramuscular implantation of M. avium suspension allergy to avian tuberculin was observed within a fortnight while within the same period histological lesions were found in muscle at the spot of puncture, in liver and spleen. Macroscopic lesions at the spot of puncture and in liver appeared in 21 days, and miliary tuberculosis of liver and spleen was detected almost regularly since day 28. Cultivation demonstrated mycobacteria in all the organs and tissues examined except the intestine in 12 days after infection. After peroral infection tuberculin allergy was observed in 106 days. Histological lesions were found in spleen in 49 days, as well as in liver, lungs and intestines in 96 days. Macroscopic lesions occurred in liver, spleen, lungs and intestine in 106 days. Small numbers of mycobacteria were isolated from various organs and tissues in 36 days after infection, and also from the intestine in 160 days. After infection by contacts with the tuberculous fowl tuberculin allergy appeared in 157 days. The first tuberculous lesions appeared in liver in 106 days, in spleen and intestine in 180 days.(ABSTRACT TRUNCATED AT 250 WORDS)