The effects of rifampicin and isoniazid on liver function have been studied in 63 patients with pulmonary tuberculosis; 29% showed abnormalities of serum aspartate aminotransferase (SGOT) and a similar percentage abnormalities of serum bilirubin. These usually occurred during the first 12 weeks of therapy. The average duration of the abnormalities was 14(1/2) days, irrespective of whether treatment was interrupted or not.The relationship between raised SGOT and acetylator phenotype in a small number of patients suggests that those with raised SGOT are usually slow acetylator phenotypes. It seems that hepatic reactions in patients with previously normal liver function are usually mild and non-specific. However, patients who continue with rifampicin should be kept under close biochemical observation.