A new method for determining cycloserine in plasma samples is described. This method is based on the derivatization of cycloserine with p-benzoquinone, a reaction that takes place at the same time as the process of plasma deproteinization due to the presence of ethanol as solvent in the solution of the derivatization reagent. Four derivatives are obtained from this reaction. The main derivative is well correlated with the cycloserine concentration. The ratio between the volumes of the plasma sample and the reagent solution is 1:2 for a p-benzoquinone concentration of 1000 microg/mL. Elution from a C18 column was isocratic, using a mobile phase containing (v/v) 85% aqueous 0.1% formic acid solution, and 15% (v/v) of a mixture of methanol and acetonitrile (1:1), with a flow-rate of 1 mL/min, at 25 degrees C. Determinations by fluorescence detection were achieved with excitation at 381 nm and emission at 450 nm, with a detection limit of 10 ng/mL for an injection volume of 5 microL. This method was validated and applied to the determination of cycloserine in blood plasma samples of several healthy volunteers.