Pulmonary surfactant is a complex mixture of lipids and proteins that lines the air/liquid interface of the alveolar hypophase and confers mechanical stability to the alveoli during the breathing process. The desire to formulate synthetic mixtures for low-cost prophylactic and therapeutic applications has motivated the study of the specific roles and interactions of the different components. All-atom molecular dynamics simulations were carried out on a model system composed of a monolayer of palmitic acid (PA) and a surfactant protein B peptide, SP-B(1-25). A detailed structural characterization as a function of the lipid monolayer specific area revealed that the peptide remains inserted in the monolayer up to values of specific area corresponding to an untilted condensed phase of the the pure palmitic acid monolayer. The system remains stable by altering the conformational order of both the anionic lipid monolayer and the peptide secondary structure. Two elements appear to be key for the constitution of this phase: an electrostatic interaction between the cationic peptide residues with the anionic headgroups, and an exclusion of the aromatic residues on the hydrophobic end of the peptide from the hydrophilic and aqueous regions.