An extractor has been developed for microporous membrane liquid-liquid extraction (MMLLE) of lipophilic xenobiotics at trace levels in biological fluids. This new construction allows the sample phase to be stirred, while the organic phase is pumped. The extractor was evaluated using human blood plasma with added organophosphate esters. The size exclusion properties of the membrane reduced lipid co-extraction by approximately 94% compared to ordinary liquid-liquid extraction. In combination with a solid-phase extraction (SPE) step, the method was shown to remove plasma lipids efficiently and thus allow gas chromatographic separation of the compounds. The clean-up method described, including the SPE step, showed a high level of reproducibility, and recoveries of between 72 and 83% were obtained for five of the organophosphate esters after a 200-min extraction period. Using this technique, triphenyl phosphate and an isomer of octyl diphenyl phosphate were detected in human plasma obtained from blood donors. The concentration of triphenyl phosphate ranged between 0.13 and 0.15 microg/g plasma.