Osmotic Concentration and Osmotic Pressure in Injectable Solutions**Research Department, Recordati Laboratorio Farmacologico S. p. A., Milano, Italy

A method for determining the osmotic pressure of injectable solutions by measuring the variations of the red-cell volume is described. By means of this method, substances of pharmaceutical interest can be classified into different groups according to their diffusibility through the erythrocyte membrane and their action upon it. It was demonstrated in vitro and in vivo that for many substances the iso-osmotic concentration is not equivalent to the isotonic concentration and that the confusion between iso-osmia and isotonia can have dangerous consequences.