<p>Rifampicin, isoniazid and pyrazinamide are all active bactericidal antituberculosis drugs. Rifampicin and isoniazid are particularly active against the rapidly growing extracellular organisms and have an intracellular bactericidal activity. Rifampicin also inhibits DNA-dependent RNA polymerase activity in susceptible cells and it has activity against slow and intermittently-growing M tuberculosis. Pyrazinamide, particularly in the acid pH environment of macrophages is active against intracellular organisms. Combination of these agents covers activity against the 3 different bacterial populations.</p>