Iturin A, extracted from the culture media of Bacillus subtilis, is an antifungal lipopeptide, the peptide cycle of which includes a D-Tyr residue in position 2. The antibiotic strength of iturin A is related to a change in the permeability of the membrane cells which leads to a leakage of K+ from the intracellular medium. Methylation of the D-Tyr residue dramatically decreases the biological activity of iturin A. Using the intrinsic fluorescence of D-Tyr we have shown that both iturin A and O-methyl-tyrosine iturin A enter the lipid membranes. When dimyristoylphosphatidylcholine vesicles contain iturin A we observe a change in the order degree of the lipid phase and an increase in the transition temperature. The methylated derivative has no effect. Two model membranes have been used to study the permeability changes induced by iturin A and O-methyltyrosine iturin A. Studying ionic permeability we have found that the conductance of a planar lipid membrane increases very much less when the lipopeptide is methylated. On the other hand, the release of carboxyfluorescein trapped in lipid vesicles is less upon addition of O-methyltyrosine-iturin A. We conclude that the Tyr residue of the peptide cycle plays a role in determining the interactions of iturin A with lipid membrane.