Abstract: A method and apparatus for the determination of blood cell osmotic fragility, wherein a solution of salt, such as sodium chloride, with an osmotic gradient decreasing with time is passed through a coiled capillary tube, positioned with its axis vertical so that the flow through the turns of the tube is substantially horizontal, and the flow follows Poiseuille's parabolic flow pattern. The blood sample is inserted in the coiled tube and, because of the flow pattern, the salt solution travels through the capillary tube much faster than the erythrocytes, so that the erythrocytes are exposed to gradually decreasing osmolarity for hemolysis. Optical monitoring is carried out as the hemoglobin is rapidly removed from the flowing stream. Optical density is recorded against time, employing a 547 nm monitoring beam, thus providing a hemolysis curve.