Free fatty acids in serum, together with representatives of a new class of oxirane carboxylic acids having blood-glucose-lowering activity, were determined simultaneously by fluorescence labelling with 4-bromomethyl-7-methoxycoumarin (Br-Mmc) followed by separation by high performance liquid chromatography. Esterification kinetics of selected compounds were studied, and separation conditions for analysis of oxirane carboxylic acids, related compounds, and a series of saturated and unsaturated fatty acids as their Mmc estes were established. Three compounds (3, 6 and 7) were determined from serum quantitatively by extraction, derivatization, and analysis by high-performance liquid chromatography. Detection limits from serum (1ml) were 50 ng/ml. Two hours after intravenous bolus injection of 10 mg/kg of compound 6 in dog, a serum level of 1.1 mg/1 of administered compound was determined.