Identification of the various blood components that adhere to dialyzer membranes may further understanding of blood-membrane interactions during dialysis. We have examined coil membranes following 6–36 h of clinical dialysis plus 60 min of saline rinsing with light, scanning electron (SEM) and transmission electron (TEM) microscopy. Deposition of BC identified by Wright stain was generalized (excluding areas of mesh-membrane contact) and increased directly with blood perfusion time. The BC identified included leukocytes, platelets, fibrin-like strands, and many free cell organelles. We conclude that many BC interact with coil membranes in spite of using heparin by the intermittent intravenous technique.