The objective of this study was to evaluate the level of particle or fiber shedding by the five types of commercially available transfusion microfilters (Bentley, Fenwal, Johnson & Johnson, Pall and Pioneer Filters). The study was performed in a class 100 laminar flow clean bench in order to minimize particulate contamination from extraneous sources. Particle-free phosphate buffered saline was flushed through each of 12 filters of each type. The effluent particles or fibers were collected on clean Millipore filters, and their number and size determined by light microscopy and by examination of photographs of the whole Millipore filters. In addition in the case of selected representative particles identification was sought by means of scanning electron microscopy. Small particles in the effluent media were electronically counted. The microfilters did not release significant quantities of particles of fibers greater than 10 micrometer at filtration rates of 300--600 cc/min. No release of any characteristic particles or fibers was observed for any of the filter types. The present study indicates that insignificant levels of particulate matter would be introduced into blood by any of the five types of blood microfilter.