The cellular source of the blue field entoptic phenomenon was investigated in two microvascular preparations using video-microscopy with lighting conditions similar to those under which the entoptic phenomenon is visualized within the human eye. In the wing of the hibernating bat, microvascular flow was simultaneously videotaped under transmission illumination at 430 nm and under unfiltered illumination. In the rat cremaster alternating observations were made using transmission illumination at 430 nm and epi-illumination fluorescence microscopy with leukocytes rendered fluorescent by intravenous Quinacrine. In both preparations, low magnification video-microscopy using 430 nm illumination produced a field of particles, which were brighter than the background, flowing within a network of dark vessels. The appearance of the particles and their movement simulated the blue field entoptic particle motion. Under higher magnification, the particles appeared brighter than the plasma gaps between red blood cells and were demonstrated to be leukocytes by morphology, by specific staining and by typical behavioral movement. The particles were observed in terminal arteriols capillaries, and post-capillary venules where they were not obscured by red blood cells. The results of this study of two microvascular preparations strongly suggest that in the human eye the blue field entoptic phenomenon is produced by leukocytes flowing within the macular retinal microvasculature.