Slow reacting substance of anaphylaxis (SRS-A) has been shown to be one of the major mediators in hypersensitive reactions and to be composed of leukotriene (LT) C4, LTD4 and LTE4. In the present study, we examined the properties of SRS-A released from sensitized guinea pig lungs by antigen and SRS released from rat peritoneal exudate cells and from human leucocytes by ionophore A23187 (0.5 and 0.2 microgram/ml, respectively). By the incubation with SRS-A, SRS and LTs with arylsulfatase (type V) in pH 5.7 buffered solution at 37 degrees C for 30 min, SRS-A and LTD4 were greatly inactivated and rat SRS was slightly inactivated, but human SRS and LTC4 were not inactivated at all. The same results were obtained when aminopeptidase was used in place of arylsulfatase. Moreover, when SRS-A, LTC4 and LTD4 were incubated with 0.02 mg/ml of gamma-glutamyltranspeptidase (gamma-GTP) pH 8.0 buffered solution at 37 degrees C for 30 min, the activities of SRS-A and LTD4 were slightly decreased, but those of SRS and LTC4 were obviously potentiated. On the other hand, incubation with a large amount of gamma-GTP (0.2 mg/ml) a dose at which this enzyme preparation showed clear aminopeptidase activity, SRS-A, SRS, LTC4 and LTD4 were obviously inactivated. In addition, we found a peak of LTD4 in guinea pig SRS-A, that of LTC4 in human SRS, and that of LTC4 in rat SRS on high performance liquid chromatograms. From these results, we demonstrated that guinea pig lung SRS-A is mainly composed of LTD4, human leukocyte SRS is mainly LTC4, and rat peritoneal SRS is composed of both LTC4 and LTD4. The inactivation of LTD4 and SRS-A by arylsulfatase may be due to aminopeptidase contamination in the enzyme preparation.