Wc have developed a highly sensitive and highly selective method tk)r the dctertnination of Leukotriene B 4 (LTB.;) in human plasma using negative ion chemical ionization/gas chromatography/tandem mass spectrometry (NICI/GS/MS/MS) analysis. The developed tuethod was summarized as follows. Deutcratcd LTB~ (d4-LTB4) was added to human plasma samples ;ts an internal standard, anti samples were extracted by a Scp-pak C18 column. Extracted L'I'B4 was dcrivatizcd into the pcntalluorobcnzyl ester of bis-trimethylsilyl ether (1)FB-TMS-LTB4) and quantilicd oil the basis of sclcctctl reaction monitoring (SRM) at m/z 299 of [M-PI:B-2TMSO! l] by NICI/GC/MS/MS analysis, which was the product ion of [M-PFB l . The detection limit for the qttantification of LTB.~ in hunlan plasma was 10 pg ml ', sufficiently sensitive to determine the concentrations of endogenous LTB.~ in human plasma. The plasma level of LTB4 rncasurcd in healthy male volunteers was 33.85 + 33.91 pg ml i (mean + S.D. in six volunteers). The technique of MS/MS used in this method ofl'crs much greater sensitivity and selectivity than single-stage mass spectrometry. The developed method showed good reproducibility with a simple and rapid extraction procedure, and would be useful for examining the relationship between various disease states and the levels of LTB.t in biological fluids.