The action of iturin A on non-growing cells of Saccharomyces cerevisiae was tested. This antibiotic gave important modifications in the membrane permeability which permitted nucleotides, proteins, polysaccharides and lipids to escape from cells. The lipid content of cells was strongly disturbed; the level of phospholipids, essentially phosphatidylcholine, decreased while the level of fatty acids increased. A part of these fatty acids were extruded from yeast cells. The role of iturin A in these modifications was discussed.