We studied the stability of blood collected into four commonly used anticoagulants at refrigerator and room temperatures. Blood collected into tripotassium or disodium ethylenediaminetetraacetate was stable for 24 hr. in the refrigerator for white and red blood cell counts, hemoglobin, hematocrit, mean cell volume, mean cell hemoglobin, and mean cell hemoglobin concentration, determined both by conventional methods and by the Coulter Counter Model S. In all four anticoagulants studied there was slight swelling of the red cells on storage at room temperature. The stability of refrigerated blood collected into ethylenediaminetetraacetate permits the use of stored patient specimens as a test material for quality control of the Model S. We have introduced a practical method, which has advantages over the use of commercial cell controls.