The magnetic characteristics of hemoglobin (Hb) changes with the binding of dioxygen (O2) to the heme prosthetic groups of the globin chains: from paramagnetic ferrous Hb to diamagnetic ferrous oxyhemoglobin (oxyHb) with reversibly bound O2, or paramagnetic ferric methemoglobin (metHb). When multiplied over the number of Hb molecules in a red blood cell (RBC), the effect is detectable through motion analysis of RBCs in a high magnetic field and gradient. This motion is referred to as magnetophoretic mobility, which can be conveniently expressed as a fraction of the cell sedimentation velocity. In this Article, using a previously developed and reported instrument, cell tracking velocimetry (CTV), we are able to detect difference in Hb concentration in two RBC populations to a resolution of 1 × 107 Hb molecules per cell (4 × 107 atoms of Fe per cell or 4-5 femtograms of Fe). Similar resolution achieved with inductively coupled plasma-mass spectrometry requires on the order of 105-106 cells and provides an average, whereas CTV provides a measurement for each cell. CTV analysis revealed that RBCs lose, on average, 17% of their Hb after 42 days of storage, the maximum FDA-approved length of time for the cold storage of RBCs in additive solution. This difference in Hb concentration was the result of routine RBC storage; clinical implications are discussed.