A series of 12 compounds, consisting of 3 known antileprosy drugs (dapsone, B663 and Ciba 1906) and 9 structural derivatives of dapsone, were tested for their antimycobacterial activity against 50 strains of M. intracellulare. Detailed investigations of B663, the only compound showing considerable in vitro activity, were carried out. All of the strains of M. intracellulare were inhibited by a B663 concentration of 1.6 .mu.g/ml and 88% were inhibited by a drug concentration of 0.8 .mu.g/ml. The rate of emergence of in vitro resistance was similar to that of M. tuberculosis against streptomycin and isoniazid. Preliminary experiments indicated that the action of the drug on M. intracellulare may be bacteriocidal in nature.