Spherocytosis is a hereditary disease. It results from mutations in genes that encode proteins participating in the attachment of the membrane skeleton to the plasma membrane bilayer of the erythrocyte. In affected cells, interaction between the spectrin-actin meshwork and integral membrane proteins is altered. This results in the weakening of plasma membrane mechanical resistance and diminishing its elasticity. Since defective cells are prone to mechanical destruction and phagocytosis in the spleen, the fraction of morphologically-altered erythrocytes is rather small; this in turn means such an examination is prone to errors. In this paper, we describe a simple method which could be useful in the identification of red blood cells with altered osmotic properties. The method is based on the measurement of the amount of light scattered by a suspension of the red blood cells, during which cells are exposed to osmotic stress in the stopped-flow regime. The obtained plots are fitted to a mathematical formula, the parameters of which can be used as quantitative indicators of the changes in red blood cells' osmotic features. Two types of spherocytotic samples were examined: those with a proven deficiency in ankyrin and those with a decrease in the band 3 anion transporting protein. The presented data show that this method gives a reliable indication of altered osmotic properties of the spherocytic cells.