15 patients underwent plasma exchange using a standard 170-μm in-line filter with or without an additional 40-μm microaggregate filter in the return circuit. The mean platelet count fall immediately after plasma exchange in 15 patients was (54 ± 6)×109/l and 48 h later was (23 ± 8)×109/l representing a mean total platelet loss immediately after plasma exchange of (253 ± 31)×109 with no difference in the 40-μm filtered procedures. The mean platelet loss in the discarded plasma was (60 ± 8)×109 and a mean of 52 ×109 platelets were recovered from machine harness washings. The platelet loss in the removed plasma and in the harness therefore accounted for only 42-44% of the total loss of platelets. The inclusion of 40-μm microaggregate filters did not reduce platelet loss, and it is therefore unlikely that the thrombocytopenia is induced by reinfused microaggregates. It is likely that platelets are activated in the machine and sequestered in the spleen.