The present work is designed to establish the effect of some hexosephosphates esters on the deoxygenation capacity of haemoglobin. To this end the dissociation curves of oxyhaemoglobin in hemolysates of human erythrocyte have been determined in the presence of increasing concentrations of hexosephosphates (2.5; 5; 10 muM/g Hb). The curves appear significantly shifted to the right in the presence of fructose-1,6-diphosphate, showing its possible behaviour as an allosteric effector on the haemoglobin molecules.