The synthesis of a tetrabiotinylated porphyrin ligand, 5,10,15,20-tetrakis{alpha-[4-(biotinylamidomethyl)-pyridinium bromide]-p-tolyl}porphyrin (TBPP), is described and its monolayer behavior at the air-water interface investigated. Monolayers of the pure salt were found to be unstable. The effects of complexing with different mole ratios of sodium octadecyl sulfate (ODS) were studied. The TBPP-ODS (1:4) complex gave stable isotherms from which it was deduced that porphyrin molecules lay flat on the water surface, with biotin moieties pointing into the subphase. From STM images it was deduced that the high deposition ratio (similar to 1.5) during vertical dipping probably arose from a spontanous self-assembly of the complex into rodlike stacks. Fluorescence microscopy showed that, in the "gaseous" phase, the complex assembled into a network of associated porphyrin molecules surrounding circular voids of different size. Addition of active avidin or streptavidin caused a significant expansion of the isotherm that was accompanied by the appearance of a domain-like structure in the monolayer. That no such changes were observed with inactive proteins confirms the specific nature of the interactions being investigated.