During the investigation of the BCG allergenic potency it is advisable to vaccinate with decreasing doses, estimating the lowest dose that induces tuberculin sensitivity and specific morphological inflammation. Although the biological test does not reveal the mathematical correlation of dose-effect relationships, it is important to look for the determination of the minimal sensitizing dose for every BCG vaccine. In this study, three groups of twenty four guinea pigs were vaccinated with decreasing doses of reconstituted BCG vaccine: 120 ng, 12 ng and 1.2 ng. Tuberculin tests were performed in different groups at the 30th, 60th, 90th and 120th day after BCG injection. The negative tuberculin reactions converted to positive between the 60th and 90th day. The dose of 12 ng elicited the largest tuberculin reactions in the animals. This dose contains 65 viable bacteria and could be regarded as the smallest effective sensitizing dose of the BCG vaccine, substrain Sofia SL222. The morphological examination demonstrated that very low inoculums (1.2 ng or 6 viable cells) were sufficient to induce a specific inflammation after BCG vaccination. The immune response in lungs and bronchus-associated lymphoid tissue (BALT) of all BCG doses applied was strongest on the day 60. In the same term, lymph nodes and spleens were characterized with blast transformed lymphoid follicles with epitheloid and Langhans giant cells even with the smallest injected dose of 1.2 ng.