We report the synthesis of a water-soluble tetrafunctional biotin ligand based on the porphyrin moiety and its behaviour at the air-water interface. The addition of streptavidin or avidin to the subphase is shown to cause a significant expansion of the isotherm of the tetrabiotinylated ligand, indicating a strong interaction between the protein and the ligand. The addition of inactive protein to the subphase caused no such change from which it is deduced that non-specific interactions could not have been responsible for the effects observed with the active proteins. Supplementary experiments using column chromatography provide evidence for the formation of a high molecular weight polymer when the tetrabiotinylated ligand and active protein are mixed.