According to the claims, the present invention comprises the preparation of chemically modified, cross-linked hemoglobins with improved functional properties, the cross-linked hemoglobins prepared according to this method and the use of these hemoglobins as artificial oxygen carriers. The synthesis method is characterized by technical simplicity as well as by high yields. Deoxygenated hemoglobin of high purity is conjugated covalently under the protection of an antioxidant with an effector of oxygen binding, especially with pyridoxal-5-phosphate, after which the hemoglobin is polymerized with glutardialdehyde as a bifunctional cross-linking agent. At the same time, there is a large increase in the volume of the reaction mixture and a decrease in the concentration of the hemoglobin during the addition of the cross-linking agent. Subsequently, after further dilution with water, a polyethylene oxide derivative is chemically linked to the cross-linked hemoglobins. Polymers are obtained, which are compatible with blood plasma and have optimized cooperativity and half saturation pressure values and can find use as artificial oxygen carriers and, in particular, are divided into a lower molecular weight fraction as blood substitute and a higher molecular weight fraction as blood additive, for example, for the treatment of oxygen-deficiency conditions.