Thirty two strains of Myc. avium showing typical or atypical cultural and biochemical properties, and 8 strains of atypical bacilli, belonging to II, III and IV Runyon's groups, were inoculated into guinea-pigs, rabbits and chickens. All the Myc. avium strains were not pathogenic for guinea-pigs but were very virulent to rabbits and chickens. The atypical bacilli were not pathogenic for all the animals to be used, but some of them induced tuberculin allergy. Myc. avium, Myc. fortuitum, strain of scotochromogenic bacilli--all isolated from pigs and Myc. avium D4, were used for study of the sensitizing properties. The experiments were performed on albinotic guinea-pigs sensitized intramuscularly and young pigs sensitized per os. The results showed that avian bacilli possessed much stronger sensitizing properties than Myc. fortuitum or scotochromogenic bacilli. Tuberculin allergy caused by atypical bacilli usually lasts for a short time. However, infection of pigs with atypical bacilli can sometimes allergical diagnosis of swine tuberculosis difficult.