Mixed monolayers of PMMA-lysozyme show the existence of negative deviations from the additivity of the molecular areas (A(m)) when the composition of polymer mixtures is less than X-PMMA 0.6, regardless of the surface pressure of the monolayers. The maximum deviation occurs in the mixed monolayer with composition X-PMMA 0.25, which is attributed to the formation of a complex consisting of one polymer molecule and three protein molecules (1:3 stoichiometry), stabilized by hydrogen bonds between the NH groups of the protein and the CO groups of the polymer as well as by van der Waals attractive proportion of the components in the mixed films significantly differs from the forces between the hydrocarbon chains of both components. When the relative value corresponding to the stoichiometry of the complex (as in mixtures with X-PMMA > 0.6), this complex cannot be formed, causing an immiscible system where the values of the experimental molecular areas coincide with those corresponding to ideal behavior. Measurements of monolayer thickness and BAM images allow confirmation on the microscopic level of the structural characteristics deduced from the pi-A isotherms.