This proposal is concerned with the determination of the chemical structures of slow-reacting substances (SRS) from cat paws and rat basophilic leukemic (RBL-1) cells, which have now been purified for the first time. The metabolism of arachidonic and 5,8,11,14,17-eicosapentaenoic acids (E.P.A.) in relation to SRS RBL formation will be investigated in RBL-1 tissue culture systems. Radioactive E.P.A. will be prepared to detect the possible existence of novel lipoxygenase pathways in tissue homogenates. PGH3 will be prepared and its decomposition products will be characterized.