Under laboratory conditions 3 types of filter designed to prevent the passage of microemboli into the circulation during the transfusion of stored blood were compared with the standard Baxter BR10 filter, designed only to hold back macroemboli, in respect of resistance to blood flow and damage to the cellular elements of the blood. In both respects the Pall Ultipor microfilter proved more efficient than the Swank IL200 microfilter and the Bentley Polyfilter (PF-127) and, except with 3-week-old blood, was comparable to the Baxter filter.