Foreign particles contaminating hemodialyzers constitute a risk of microembolism and allergic reactions in hemodialysis patients. We investigated the size distribution of particles, and the effects of striking headers of dialyzers and flow rates of rinsing saline on the elimination of foreign particles from dialyzers. Saline used for rinsing was filtered through a Millipore filter, and the particles thus removed were observed microscopically. We also checked changes in particle counts in a closed circuit consisting of a dialyzer and blood lines during 5 h of continuous circulation with a blood pump. The number of contaminating particles increased exponentially as their size decreased. For the sake of reproducibility we counted particles having diameters larger than 3.0 μm to monitor the rinsing efficiency. During rinsing, particles in the dialyzer were effectively eliminated by striking the headers of the dialyzer. We found that striking should be started immediately after the blood line is filled with saline, and that variation in saline flow rates in the range of 350–700 ml/min of saline does not affect the rinsing efficiency. By filtration of saline used for rinsing, particles, mostly ranging from 5 to 200 μm in length, of many shapes and colors were “found. Among them were fibers having a length of as much as a few millimeters. Once dialyzers were rinsed effectively, there was almost no change in particle counts in a closed circuit consisting of a dialyzer and blood lines. To rinse dialyzers effectively, at least 1,000 ml of saline are necessary, and striking the headers of dialyzers throughout the rinsing procedure is important.