Iturin A and bacillomycin L, antibiotics of the iturin group inhibit the growth of Saccharomyces cerevisiae and the lethal doses were respectively 10 and 60 μg/ml. Both antibiotics had an effect on the incorporation of radioactive precursors into macromolecules which decreased with increasing concentrations of antibiotics. However, no specificity was observed on the various macromolecules, proteins, ribonucleic acids and polysaccharides. The site of action on yeast cells was demonstrated to be the cytoplasmic membrane: both antibiotics of iturin group lysed spheroplasts of S. cerevisiae. Moreover, a rapid leakage of potassium ions occurred in the presence of the antibiotics; this leakage was directly associated to the killing effect. These results are consistent with a disruption of the structural integrity of the cytoplasmic membrane correlated to the loss of viability of the yeast cells.