Dynamic behavior of monolayers of a biodegradable polymer (poly(D,L-lactic acid), PLA50) and a protein (bovine serum albumin, BSA) spread at the air/water interface are studied to clarify the organization of the films with regard to the composition and the state of compression. A theological approach based on the Maxwell’s model was found to be useful to discriminate the influence of each constituent in the monolayer. Before the transition phase of PLA50, theological properties of the mixed monolayer are governed by the PLA50 which constitutes probably the continuous phase of the system. During the transition phase of PLA50, condensation of the PLA50 segments allow the formation of a continuous phase mainly constituted from BSA. After the transition phase of PLA50, theological properties of the mixed monolayer were shown to be identical to the properties of a pure BSA monolayer. This indicates that BSA is present between PLA50 aggregates and prevents the connection of these domains at least until the higher studied pressure (i.e. 21 mN/m). The obtained results suggest that BSA could be an efficient stabilizer of the interfacial region during preparation of PLA50 particles by an emulsion technique.