The penetration dynamics of bovine beta-lactoglobulin dissolved in a buffered aqueous substrate into a fluidlike Langmuir monolayer of dipalmitoylphosphatidylcholine (DPPC) are experimentally and theoretically studied. The beta-lactoglobulin penetration induces a first-order phase transition in the fluidlike Langmuir monolayer indicated by an inflection point in the pi(t) pentration kinetics curves and visualized by BAM. After the inflection point, which corresponds to the phase transition point, condensed phase domains similar those of pure DPPC are formed. The number and growth of the domains depend on the area per one DPPC molecule at which the beta-lactoglobulin penetration takes place. The experimental results suggest that the adsorbed protein induces the condensation of the pure DPPC component. A theoretical model is proposed that supports the experimental findings. The equation of state for mixed monolayers are derived for the conditions that at the beginning aggregation to condensed phase structures does not occur, that it is induced by the penetrated species, and that the condensed phase consists only of the insoluble monolayer component. The values calculated for the surface pressure, the critical surface concentration for aggregation to condensed phase and the time when the aggregation commences, are in satisfactory agreement with the experimental data.