Abstract A haemolytic method has been used for the determination of the isotonic concentrations of dextrose, fructose, galactose and mannose. Fructose, galactose and mannose produce haemolytic effects deviating from those of dextrose. The isotonic concentration in mM per cent of fructose was found to be 20.3; of galactose, 38.9; of mannose, 33.3; whereas that of dextrose is 26 mM per cent.