LTE4 is produced by the action of a dipeptidase on LTD4 leaving only the cysteinyl group still attached to the fatty acid backbone.{2236} It is one of the constituents of slow-reacting substance of anaphylaxis (SRS-A).{394} LTE4 is considerably less active (8 to 12-fold) than LTC4 in the biological activities characteristic of cysLTs.{2236,396} 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.{1815,865,2064} LTE4 methyl ester is a more lipid soluble form of LTE4. The biological activity of LTE4 methyl ester has not been reported.