Three peptides, Lac21, Dan25, and DN1, were tested for their ability to form force-transmitting networks at the air-water interface. Lac21. is known to be highly surface active and adopts the commonly occurring a-helix motif following self-assembly with other Lac21 peptides. Mechanical testing of Lac21 adsorbed at the air-water interface demonstrated a fluid interfacial layer incapable of transmitting a force laterally in the plane of the interface; no network formation was observed, and consequently the interfacial elasticity modulus and the maximum interfacial stress were approximately zero. The second peptide, Dan25, was designed by making conservative changes to the Lac21 sequence to include cross-linking functionality. The interfacial elasticity modulus (71.9 mN/m) and maximum interfacial stress (15.0 mN/m) were higher than reported previously for beta-lactoglobulin and beta-casein. Further increases in both parameters (to 93.6 and 18.0 mN/m, respectively) were possible through formaldehyde cross-linking, although this caused a significant shift in the shape of the stress-strain plot, suggestive of a significant change in material properties due to the enhanced cross-linking. This result indicates the extreme impact that relatively minor changes in peptide sequence can have on interfacial network properties and emphasizes the need to separate interfacial energy and network formation effects when characterizing interfacial protein and peptide films'. The third peptide, DN1, has previously been shown to self-assemble in bulk solution to form beta-sheet-based filaments. The DN1 peptide showed a time-and concentration-dependent force response, probably as a result of complex interfacial self-assembly. The mechanical properties of a self-assembled DN1 film can be altered through careful control of concentration and adsorption time, although this process is more difficult to control than when helical-derived peptides are employed..