Abstract: An automatic monitoring and recording device for erythrocyte sedimentation employs a light source and a photodetector to detect the changes of light transmittance at the junction of plasma and red blood cell column. Changes of light transmittance at the junction reflect the descent of red blood cell column. The light source and photodetector pair is arranged so that it will track to the plasma - red blood cell column junction all the way the red blood cells fall. The whole course of erythroctye sedimentation is monitored and recorded automatically by a microcomputer.