Surface-active properties including dynamic adsorption, monolayer stability, micelle forming capacity, and foaming aptitudes of surfactin-C-15/iturin A-C-15 mixtures were studied. Surfactin-C-15 and iturin A-C-15 molecules interact in synergism on the most surface-active properties evaluated at 20 degrees C at the air-water interface and in aqueous solution (pH 8.0). The synergism is positive on the adsorption effect, monolayer stability, foam density, and liquid stability in foam, whereas it is negative on the adsorption rate. No synergism occurs an micelle forming capacity, but surfactin-C-15 and iturin A-C-15 form mixed micelles when the solution contains a low proportion of surfactin-C-15.ses the synergistic effect is maximum when surfactin-C-15 and iturin A-C-15 molecules are mixed in a 2:3 ratio. This is attributed to the surfactin/iturin A complex formation resulting from specific interactions among two surfactin-C-15 molecules and three iturin A-C-15 molecules. A model of such a complex formation is proposed.