The activity of pyrazinamide (PZA) against eight isolates of Mycobacterium tuberculosis in a murine infection model was evaluated. M. tuberculosis isolates with various degrees of in vitro susceptibility to PZA (MIC range, 32 to > 2,048 micrograms/ml) were used. Four-week-old female mice were infected intravenously with approximately 10(7) viable M. tuberculosis organisms. PZA at 150 mg/kg of body weight was started 1 day postinfection and given 5 days/week for 4 weeks. Infected but untreated mice were compared with PZA-treated mice. Mice were sacrificed at the completion of the treatment period, and viable cell counts were determined from homogenates of spleens and right lungs. PZA had activity in the murine test system against M. tuberculosis isolates for which the MICs were < or = 256 micrograms/ml. However, there was an inconsistent correlation between the absolute MICs and the reductions in organ viable cell counts. Studies with drug-resistant M. tuberculosis isolates with an isogenic background would improve evaluation of drug efficacy in the murine test system. Further evaluation of antimycobacterial agents against monodrug-resistant isolates will provide data that will be useful for development of algorithms for treatment of infection with drug-resistant organisms.