Interest in determining plasma levels of rifampin for adjustment of dosage regimens has increased, but conflicting results exist concerning rifampin stability. The authors developed a high-performance liquid chromatography assay to monitor rifampin plasma concentrations that was used to study the possible degradation of rifampin in plasma samples. This report describes the stability of rifampin in plasma kept at an ambient temperature for 24 hours or stored at -20 degrees C for 2 weeks. The possible protective effect of adding ascorbic acid was also studied. The results indicate that rifampin degrades rapidly in plasma at an ambient temperature, and a 54% loss was observed within 8 hours. This degradation can be effectively prevented by adding ascorbic acid, thus prolonging stability for up to 12 hours. The same results were observed with samples obtained as part of routine drug monitoring. Degradation was found to be greater at low rifampin concentrations. The authors subsequently demonstrated that decomposition of rifampin occurs after storage for 1 week at -20 degrees C. However, in samples supplemented with ascorbic acid before freezing, no degradation was observed within 14 days at the two concentrations tested. Rifampin was more stable in specimens drawn from treated patients, suggesting possible in vivo stabilization of the molecule. Further studies are needed to determine stability of rifampin for longer storage periods. On the basis of these results, plasma samples obtained from patients receiving rifampin should be immediately supplemented with ascorbic acid and analyzed as soon as possible.