The use of the antifungal agent flucytosine has been associated with dose-related hepatotoxicity and bone marrow depression, and measurement of plasma concentrations is useful in determining dosage schedules. The present study describes a rapid, simple, and sensitive high-pressure liquid chromatographic analysis of plasma flucytosine which may be completed within 30 min of receipt of a plasma sample. Sample preparation is minimal and merely involves precipitation of plasma protein before injection onto a reverse-phase column. The method is accurate and reproducible and yields results comparable to or perhaps slightly superior to those obtained by conventional microbiological assay. The value of the procedure for routine use is enhanced by the freedom from interference by a large range of medications administered concurrently with flucytosine.