Black tuberculosis patients from South Africa (S. A.) as well as from Birmingham, Alabama, U.S.A., showed a higher percentage of fast inactivators of isoniazid (INH) than that found in the North American white population, simultaneously sampled. In S. A. blacks, the frequency of fast inactivation was 57.9--59.6%, while in American blacks of Birmingham it amounted to 60.3%; in comparison to the above groups the rate of fast acetylators in Canadian Caucasians was 41.9% and in the USA white population 41.0%. For phenotyping of isoniazid inactivators a urine test was used. In this method the concentrations of INH (including isoniazidhydrazones) as well as acetylisoniazid were determined in the specimens collected 6--8 hrs following a test dose of 10 mg/kg INH.