This International Conference on Atypical Mycobacteria, a gathering of outstanding scientists of many fields, attests to the biologic and clinical importance of the mycobacteria and mycobacterioses. This conference may help provide a basis for the formulation of cooperative projects for the attainment of knowledge in each of several fields where I believe important information is lacking and needed. Mycobacteria exhibit a fascinating diversity. It is well to come to some understanding with regard to nomenclature. The word atypical as used here does not mean atypical-it means MOTT (mycobacteria other than tubercle bacilli). MOTT are not atypical mycobacteria; neither are they atypical tubercle bacilli. Some individuals use the word opportunistic to describe MOTT. This description is not objectionable; tubercle bacilli, of course, are opportunists. Most persons who become infected with friendly tubercle bacilli do not become obviously diseased, but the bacilli become pathogenic if opportunity permits. As for species names for MOTT, these should be designated according to the International Code of Nomenclature of Bacteria [1]. No one is forced to follow the Code, but by doing so, one and only one universally recognized designation for each taxon is achieved. Correct species names of mycobacteria are given in the 1980 approved lists [2]. Agreement on the meaning of the word tuberculosis is highly desirable. Tuberculosis is a disease caused by any tubercle bacillus but not by any other species. Tubercle bacilli include Mycobacterium tuberculosis, Mycobacterium bovis, and Mycobacterium africanum. Together with Mycobacterium microti (not pathogenic for humans) these organisms constitute the tubercle bacillus complex. Similarly, tuberculins are derived only from tubercle bacilli, not from other species of mycobacteria. Abbreviations for mycobacte-