The effect of ioxitalamate, a radiological contrast medium, on red blood cell (RBC) elastic properties has been studied. Individual red blood cells are gently aspirated by an accurately controlled pressure into the tip of a glass micropipette. Cells are deformed with a well‐defined force produced by a very small suction pressure. An elasticity shear modulus is then deduced.At a concentration isoosmotic to plasma, the elasticity shear modulus remains normal. At a concentration hyperosmotic to plasma the elasticity shear modulus is significantly higher than the one obtained for cells suspended in either isoosmotic or hyperosmotic buffer.It can be concluded that the effect of ioxitalamate at a volume ratio of 20% on elastic properties of RBC is related to membrane changes and not to the increase in osmolality.