The slow-reacting substance of anaphylaxis or SRS-A is a mixture of the leukotrienes LTC4, LTD4 and LTE4. Mast cells secrete it during the anaphylactic reaction, inducing inflammation.^[1] It can be found in basophils.

It induces prolonged, slow contraction of smooth muscle and has a major bronchoconstrictor role in asthma.^[2] Compared to histamine, it is approximately 1000 times more potent and has a slower onset but longer duration of action.^{[citation needed]}