An HPLC assay was developed to determine the paclitaxel analogue 13-(N-tert.-butoxycarbonyl-beta-isobutylisoserinyl)-14-hydroxyb accatin-1,14-carbonate (IDN5109) and its epimer in mouse plasma. The method involves solid-phase extraction on cyano cartridges (recovery >75%), HPLC separation on symmetry shield column, a mobile phase of NaH2PO4 (10 mM) pH 5.2, acetonitrile (47:53) and detection at 227 nm. Retention times of IDN5109, its epiform and internal standard were 15, 24 and 25.5 min, respectively. The assay was linear from 0.10 to 10 microg/ml (r2 = 0.999), with a C.V. <5% and accuracy in the range of 95-107%. LOQ was 50 ng/ml for both compounds. Using this method IDN5109 pharmacokinetic was determined in mice.