Sixteen guinea-pigs divided into two groups were infected with cultures of Mycobacterium bovis and Mycobacterium avium. In each group the experimental animals were divided into four subgroups, and the two guinea-pigs in each subgroup were subjected to peroral, intramuscular, subcutaneous or intraperitoneal. infection, respectively. Within a period of 20 days to 3 months all guinea-pigs in the group infected with M. bovis died. During the whole period of the investigation (five months) all guinea-pigs in the group infected with M. avium remained alive. Bifragment amplificates in size of 600bp and 320bp were obtained from internal parenchymatous organs of the animals infected with M. bovis, and monofragment amplificates in size of 600bp were obtained from the guinea-pigs infected with M. avium. Pathohistological characteristics of the changes in the internal organs was presented in details.