LTE4 is produced by the action of a dipeptidase on LTD4 leaving only the cysteinyl group still attached to the fatty acid backbone. It is one of the constituents of slow-reacting substance of anaphylaxis (SRS-A). LTE4 is considerably less active (8 to 12-fold) than LTC4 in the biological activities characteristic of cysLTs. 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. LTE4 methyl ester is a more lipid soluble form of LTE4. The biological activity of LTE4 methyl ester has not been reported.