The lipid composition affects Trastuzumab adsorption at monolayers at the air-water interface.  Trastuzumab (Tmab), an antibody for breast cancer, was incorporated in Langmuir monolayers with different lipidic compositions to investigate the drug action in lipidic interfaces of pharmaceutical interest. Tmab caused all lipid films to expand as confirmed with by surface pressure-area isotherm, proving its incorporation. It also affected the compressional and structural properties as observed by in-plane elasticity curves and polarization modulation reflection-absorption infrared spectroscopy (PM-IRRAS), respectively. Although Tmab did not change significantly the compressional modulus for dipalmitoylphosphatidylcholine (DPPC) monolayers, it decreased it for the mixtures of DPPC with cholesterol. In contrast, for dipalmitoylphosphoethanolamine (DPPE), Tmab increased the compressional modulus for both monolayers, pure DPPE or mixed with cholesterol. While Brewster Angle Microscopy showed discrete distinctive morphological patterns for the monolayers investigated, PM-IRRAS showed that Tmab caused an increased number of gauche conformers related to the CH2 stretching mode for the lipid acyl chains, suggesting molecular disorder. Furthermore, the antibody kept the β-sheet structure of the polypeptide backbone adsorbed at the lipid monolayers although the secondary conformation altered according to the film composition at the air-water interface. As a result, the results suggest that the membrane lipid profile affects the adsorption of Tmab at lipid monolayers, which can be important for the incorporation of this drug in lipidic supramolecular systems like in liposomes for drug delivery and in biomembranes.