The specific interaction at the air-water interface between streptavidin and a monolayer of the tetrabiotinylated ligand 5,10,15,20-tetrakis (α-[4-(biotinylamidomethyl)pyridinium bromide]-p-tolyl} porphyrin, stabilised by complexation with sodium octadecyl sulphate, is observed directly by surface pressure measurements and by fluorescence microscopy. Changes in the structure of the monolayer, especially the appearance of domains, after adding protein to the subphase confirm that the concomitant expansion of the pressure-area isotherm is caused by the specific adsorption of protein to biotin ligands in the monolayer.