Monitoring of contaminating platelets, granulocytes, and lymphocytes in leukocyte-poor red blood cell concentrates is usually done by counting in an electronic particle counter. Sensitivity and specificity of this technique are compromised by the contamination of the preparations with other cell types and particles thereof. In this report we studied platelet contamination in filtered red blood cell concentrates by use of a radioimmunoassay for detection of the platelet glycoprotein complex IIb-IIIa. Our results indicate that platelets and/or fragments thereof, not detectable for particle counters, are present in blood cell concentrates. This finding might have important implications for the preparation of pure red blood cell concentrates to avoid unwanted immunization after transfusion.