AbstractThe interaction of heparin with salmine protamine was investigated using the electrostatically bound fluorescent probes acridine orange and eosin Y and the 1‐(dimethylamino)‐5‐naphthalene sulfonyl group which is covalently bound to the protamine molecule. Dye displacement experiments indicate a 1 : 1 pairing of anionic heparin sites with cationic protamine sites. Further binding of heparin occurs, after all the cationic sites of protamine are bound up with heparin. This is thought to occur as a result of the different affinities of the sulfate and carboxyl groups of heparin for guanidino groups; the sulfate groups binding preferentially to the complex releasing some of the bound carboxyl groups. Fluorescence polarisation measurements suggest a heparin‐protamine complex greater in volume than would be expected from a simple pairing of heparin and protamine molecules, indicating some degree of cross‐linking. The limiting salt concentrations for the heparin‐protamine complex were determined in the prescence of a number of Group IA and IIA meta