slow-reacting substance of anaphylaxis (SRS-A) a lipoprotein of low molecular weight composed of leukotrienes; it is released in anaphylactic shock and produces slower and more prolonged contraction of muscle than histamine does; it is active in the presence of antihistamines (but not epinephrine) and is newly generated after unpreformed mast cell generation; it induces the effect observed in anaphylactic reactions. Cf.: peptidyl leukotrienes SYN: slow-reacting factor of anaphylaxis.