Magnetic fields are frequently used in medical diagnosis and therapy. Therefore, it has become of great interest to study the effects of magnetic fields on human biological systems, in particular the blood. Several studies of the magnetic behavior of blood were conducted but the results are often fragmentary and inconsistent due to the narrow range of temperatures and magnetic fields at which these studies were performed. This study investigates the magnetic behavior of human erythrocytes under extreme conditions of high magnetic fields, low temperatures, and different oxidation states of iron in the blood. The magnetization of erythrocytes was measured in the temperature range 2-100 K and in magnetic fields up to 9 T. Magnetic measurements have shown that erythrocytes with high ferric iron (Fe
) content possess higher saturation magnetization than normal erythrocytes. In this regard, erythrocytes treated with sodium nitrate (NaNO
) to achieve maximum concentrations of Fe
exhibited the highest magnetization. Erythrocytes treated with carbon monoxide (CO) revealed significant diamagnetic behavior and showed a major reduction in their saturated magnetization. For all measured samples, the magnetization approaches saturation only at temperatures close to 2 K. It is believed that NaNO
and CO affect differently the overall oxygen contents and the magnetic moments of the Fe-ions in blood.