METHODS Mycobacteriology Laboratory, National Jewish Medical and Research Center, Denver, Colorado. OBJECTIVE To evaluate the antimicrobial activity of pyrazinamide against Mycobacterium tuberculosis in cultured human monocyte-derived normal and activated macrophages. METHODS Monocytes separated from human blood were incubated in plastic plates for seven days to mature into macrophage monolayers. After activation with TNF-alpha or IFN-gamma or without prior treatment, the macrophages were infected with M. tuberculosis. Various concentrations of pyrazinamide (PZA), morphazinamide (MZA) or isoniazid (INH) were added the next day, and the viable counts of the intracellular bacteria were determined at days 0, 4, and 8. RESULTS No inhibitory activity of PZA at any concentration was detected, while clear dose-dependent bacteriostatic and bactericidal activities were demonstrated by MZA and INH in the same experimental model. CONCLUSIONS PZA has neither bacteriostatic nor bactericidal activity against M. tuberculosis persisting or multiplying in cultured monocyte-derived human macrophages, and it might be that the well-known effectiveness of this drug in tuberculosis patients is not related to its supposed activity against intracellular bacterial subpopulations.