A rapid gas chromatographic assay has been developed for the separation and determination of khellin in human serum. Using a DB-17 capillary column and a simple chloroform extraction, khellin and an internal standard, trioxsalen, were separated from endogenous substances without further clean-up. Spiked serum samples in the range 0.11-1.1 micrograms/ml were assayed and a linear calibration curve obtained.