WIN 33377 (I) is a member of a novel class of cytotoxic antitumor agents, 4-aminomethyl thioxanthone derivatives. A simple, rapid and reproducible method has been developed for the assay of I in mouse plasma using a high-performance liquid chromatographic method utilizing a column-switching technique. The method involves direct injection of buffered plasma to the extraction column for sample clean-up followed by switching onto an analytical column for analysis with UV detection at 256 nm. The method has demonstrated accuracy and precision over the range 10-2500 ng/ml using a 100-microliters plasma sample with a minimum quantifiable level at 10 ng/ml. Stability of mouse plasma samples was demonstrated after storage for 4 weeks at -15 to -20 degrees C, as was the ability of samples to be accurately quantified after a maximum of three freeze-thaw cycles. Recovery was greater than 87% for the compound and the internal standard. The assay was accurate and reproducible with measured values lying within the limits of defined acceptance criteria. The utility of the method was demonstrated by analyzing plasma samples obtained from mice dosed with I as part of a pre-clinical safety study intended to assist in the design of a pharmacokinetically guided dose escalation strategy.