A rapid sensitive method for the quantification of in vitro HIV-protease activity has been developed on the basis of the endoproteolytic conversion of N-Dns-SQNYPIV to N-Dns-SQNY. The use of the N-dansyl group as a fluorescence label was shown to not significantly alter the apparent kinetic parameters for the peptideenzyme interaction. Using fluorescence detection, the dansylated product and unconverted substrate are detected in a single rapid (3 min) isocratic reverse-phase HPLC separation in quantities as low as 0.2 pmol. The method is highly reproducible and suited to a variety of applications including the analysis of large sample numbers and rigorous enzymological studies.