It has been reported that leukotriene B4 can translocate calcium across model membranes (Serhan et. al., (1982) J. Biol. Chem., 257: 4746). Such ionophoretic behavior could account for its biological effects. We have examined the effect of chromatographically pure leukotriene B4 on Ca2+ permeability when added exogeneously at 3 μM to phosphatidylcholine liposomes and when incorporated at 5 mole % in the lipid mixture used to prepare liposomes. No effect was observed with either procedure. An oxidized preparation of leukotriene B4 stimulated calcium permeability, however, suggesting that oxidation may account for the previously reported ionophoretic behavior of leukotriene B4. © 1987.