The possible existence of kinetic and/or metabolic interactions between Rifampicin and Isoniazid was investigated in man; the effect of the concomitant presence of an impaired liver function was also studied. The results have shown that both in normal subjects and in patients with impaired liver function the kinetics and metablism of each drug given alone are not significantly different from that observed when the other drug is associated. The serum levels of each drug were significantly higher in patients with chronic liver impairment.