Two mature male beagles were infused intravenously with N hydroxysuccinimido (NHS)-biotin (35 mg/kg body weight in dimethyl sulfoxide) to biotinylate all circulating RBC (99% each), as described.9 One dog (794) was recovering from an idiopathic anemia and had a low normal hematocrit of 37% at labeling that increased to 52% by day 48, resulting in a partial cohort effect. The other dog (331) was assessed to be completely normal. Blood was taken for density gradient centrifugation at roughly 2-week intervals initially and then weekly from day 86 to 105 to determine the proportions of B-RBC in each density fraction (by flow cytometry) with increasing mean cell age. Fraction 4 contains the densest 1% of RBC.