Serum haloperidol levels were studied in schizophrenic patients with and without antituberculosis therapy, and the effect of these agents on serum haloperidol level was evaluated. Rifampicin caused significant suppression of serum haloperidol levels in all cases studied (n = 7). The serum haloperidol clearance rate was accelerated in patients taking rifampicin, with a shortened half-life (4.9 h) compared with the control group (9.4 h). Among 18 schizophrenic patients receiving isoniazid, three showed significantly elevated serum haloperidol levels. It is possible that isoniazid can elevate haloperidol levels in some patients depending upon some unknown factors. The elevation of serum haloperidol level was though to be due to prolonged clearance of haloperidol secondary to isoniazid interaction with hepatic enzymes involved in drug metabolism. These observations suggest that haloperidol doses in schizophrenic patients receiving rifampicin or isoniazid must be carefully monitored.