Interleukin-1 (IL-1) is produced and released by various cells, including activated macrophages, and plays important roles in inflammation as well as immune responses. Since the precursor of IL-1 has no signal peptide, the mechanism of IL-1 release has been an enigma. To investigate the possibility of direct interaction of IL-1 with the lipid bilayer, the interleukin-1 alpha (IL-1 alpha)- or beta (IL-1 beta)-induced permeability change of the liposomal membrane was determined. IL-1 alpha, but not IL-1 beta, caused an increase in the permeability of liposomes composed of phosphatidylserine (PS), at neutral and acidic pHs, as demonstrated by measuring the efflux of calcein. On the other hand, liposomes composed of phosphatidylcholine (PC) showed no increase in permeability when incubated with IL-1 alpha, suggesting the importance of acidic phospholipids in the interaction of IL-1 alpha with the membrane. Furthermore, permeability of liposomal membrane was markedly increased by IL-1 alpha in the presence of 1 microM calcium ions, although a permeability change was observed even in the absence of calcium ions. IL-1 alpha also induced the efflux of fluorescent dextran (average M(r) of 39,600), raising the possibility of translocation of IL-1 alpha through the cell membrane.