The effect of a rhamnolipid biosurfactant on pulmonary surfactant is studied employing the black foam film method. Pulmonary surfactant is modeled by a commercially available lung surfactant preparation (LSP). The effect of rhamnolipid concentration on the formation and stability of films formed from mixtures of LSP and rhamnolipids is experimentally studied by measurements of the probability W of formation of black foam films as a function of both LSP and rhamnolipid concentrations at the physiologically relevant electrolyte concentration C-el = 0.15 mol dm(-3) NaCl. The obtained curves show that addition of rhamnolipid at a concentration C-RhL = C-c (critical concentration of black foam film formation) to LSP suspensions causes destabilization of the foam films. In this case, additional quantities of lung surfactant preparation are needed to obtain black films with probability W = 100%. Rhamnolipid adsorption and formation of mixed adsorbed layers at the solution/air interfaces of foam films formed from mixtures of lung surfactant and rhamnolipids are experimentally studied by monitoring the effect of electrolyte and rhamnolipid concentrations on the thickness h of the foam films. The incorporation of rhamnolipid ions in the adsorbed layers at the film interfaces is evidenced also by direct measurements of the disjoining pressure Pi in the films. The Pi(h) isotherms demonstrate that the added rhamnolipids change the surface electric parameters of the films and their thickness and stability at higher pressures. The obtained results show that the different molecular components in the mixture and the increased surface charge at the film interfaces originating from the rhamnolipid ions have a significant effect on the surface forces operative in the studied films.