Hemoglobin conjugates useful as a hemoglobin-based oxygen carriers are prepared by reacting hemoglobin with oxidatively ring-opened polysaccharides such as hydroxyethyl starch or dextran, and storing the resultant conjugate under conditions which allow it to transform to a lower molecular weight product, after conjugation. The conjugate is then reductively stabilized to form secondary amino bonds between the hemoglobin and the polysaccharide, and formulated as an BOC.