Methods are described for the examination of mycobacteria cultured from clinical specimens. In the "screening" procedure used for new isolates tubercle bacilli are non-pigmented, do not grow at 25 degrees C and are sensitive to p-nitrobenzoic acid as well as normally to anti-tuberculosis drugs. Classification is extended when necessary by the use of four tests--temperature requirements, pigmentation, oxygen preference and Tween hydrolysis. These define 15 species or groups meeting the needs of clinical bacteriology. Drug-sensitivity tests are described which relate the end-points of titrations to the modal response of normal wild strains of M. tuberculosis. They are used not only as a guide to chemotherapy but also to support and amplify classification.