We describe a high-performance liquid chromatographic method for the quantitative determination of gentamicin in serum. The antibiotic was separated from serum by passage through a silicic acid column, derivatized with o-phthalaldehyde, and eluted with ethanol. The derivatized gentamicin was then separated into all three of its major components by reversed-phase chromatography and quantified by fluorometry. Concentrations in serum as low as 0.5 mg of gentamicin per liter could be accurately determined. A standard curve showed a linear response for serum containing gentamicin at concentrations ranging from 0 to 20 mg/liter. Tobramycin, amikacin, ampicillin, penicillin G, methicillin, carbenicillin, chloramphenicol, clindamycin, and cephalothin did not interfere with the gentamicin assay. Comparison with an accepted microbiological assay yielded a correlation coefficient of 0.99. This chemical assay is rapid (less than 30 min), sensitive, accurate, specific, and appears to be applicable to other aminoglycosides.