We report a model of chronic Pseudomonas aeruginosa osteomyelitis in the rat that was reproducible, simple and inexpensive. No promoting agent was required to cause infection. Infected animals yielded consistent pseudomonal colony counts (log): 4.98 +/- 0.32 (SD)/g cortical tibia (n = 16). The 95% confidence interval of the mean was 4.83-5.14. The inoculum required to infect 50% of challenged rats (ID50) was log 4.0; the ID 100 was log 6.4. Ceftazidime (50 mg/kg/8 h, subcutaneously), alone and in combination with tobramycin (40 mg/kg/12 h, subcutaneously), produced no significant change in quantitative bacterial count or gross bone pathology when used to treat established disease.