Leukotriene E4-d5 (LTE4-d5) is intended for use as an internal standard for the quantification of LTE4 by GC- or LC-MS. LTE4 is produced by the action of dipeptidase on LTD4, leaving only the cysteinyl group still attached to the fatty acid backbone.¹ It is one of the constituents of slow-reacting substance of anaphylaxis (SRS-A).² LTE4 is considerably less active (8 to 12-fold) than LTC4 in the biological activities characteristic of cysteinyl leukotrienes.^1,3 Unlike LTC4 and LTD4, LTE4 accumulates in both plasma and urine. Therefore, urinary excretion of LTE4 is most often used as an indicator of asthma.^4,5,6 In humans, basal levels of LTE4 range from 1-100 pg/mg creatinine. In asthmatic patients, urinary LTE4 levels increase to 80-1,000 pg/mg creatinine.⁵