Simple instrumentation and procedure with stopped flow system coupled to hydrodynamic injection and spectrophotometric detection were developed for automatic osmotic fragility test (OFT). OFT is a test for abnormal red blood cells based on the kinetics of their rupturing in a hypotonic saline solution. A portion of red cells was merged on-line with a hypotonic saline solution. They were mixed while flowing into the detector and were stopped for a short period for continuously monitoring the change of turbidity based on a transmission signal. In this work, a possible application of the system for screening of beta-thalassemia trait is demonstrated. Descriptions of instrumentation development and parameter optimization are presented. The system offers advantages over the conventional batch-wise OFT in terms of automation, precision, analysis time and sample volume. 2009 © The Japan Society for Analytical Chemistry.