Seventy-five patients who developed mild hepatic reactions (serum transaminase concentrations of 45 to 149 units per liter) and 50 patients who showed more serious liver damage (serum transaminase values greater than 150 units per liter) were compared with 261 consecutive patients who had no liver reactions during treatment with rifampin and isoniazid. Generally, liver toxicity occurred in 18 per cent of patients receiving combined anti-tuberculous drug therapy. Small increases in transaminase occurred in 14 per cent of the patients; large increases occurred in 4 per cent. Elderly women comprised a risk group. Among patients exhibiting a more serious hepatic lesion (transaminase values greater than 150 units per liter), alcoholics, mostly men, formed another risk group, together with other patients with a history of previous liver or biliary disease. Of 261 patients who did not develop a liver reaction, 57 per cent were slow INH acetylators. In this study, the groups with small and large increases in transaminase were clearly separated; in the former group there was no preponderance of phenotype, whereas in the latter group, slow acetylators clearly dominated among early (first 4 weeks of treatment) hepatic reactions (P less than 0.01). Studies of single-drug regimens of isoniazid have shown that neither slow nor rapid acetylation has any causal influence on isoniazid-induced hepatitis. Because the metabolism of rifampin is independent of the acetylation process, rifampin and isoniazid in combination seem to cause a toxic hepatitis that differs from the hepatitis induced by either drug separately.