Summary Porcine leukocytes incubated with an isoenzyme of phospholipase A 2 (PLA 2) (isolated from snake venom) produced several trihydroxytetraene- containing compounds which were derived from endogenous sources of arachidonic acid. The formation of these compounds was dose-dependent with an EC 50 of approximately 1.25 × 10 −8M. At this concentration of the isoenzyme and time of explosure the cells remained viable as determined by the exclusion of trypan blue. The compounds were purified by HPLC and their identities were determined by physical criteria which included U.V. spectrometry, GC/MS and by comparison with both synthetic and authentic materials. The biologically derived compounds proved to be lipoxin B (5S, 14R, 15S-trihydroxy-6, 10, 12-trans-8-ciseicosatetraenoic acid) and its two structural isomers (8-trans-LXB and 14S-8-trans-LXB). Of interest, only small amounts of lipoxin A and its isomers were found in these incubations. Results of the present study indicate that porcine leukocytes can generate lipoxin B and its isomers from endogenous sources of arachidonic acid. Moreover, they suggest that certain PLA 2 isoenzymes may initiate the formation of lipoxins and related compounds.