Human hemoglobin (Hb) conjugated to benzene tetracarboxylate substituted dextran produces a polymeric Hb (Dex-BTC-Hb) with similar oxygen affinity to that of red blood cells (P 5 0 =28-29 mm Hg). Under physiological conditions, the oxygen affinity (P 5 0 ) of Dex-BTC-Hb is 26 mm Hg, while that of native purified human HbA 0 is 14 mm Hg, but it exhibits a slight reduction in cooperativity (n 5 0 ), Bohr effect, and lacks sensitivity to inositol hexaphosphate (IHP), when compared to HbA 0 . Oxygen-binding kinetics, measured by rapid mixing stopped-flow method showed comparable oxygen dissociation and association rates for both HbA 0 and Dex-BTC-Hb. The rate constant for NO-mediated oxidation of the oxy form of Dex-BTC-Hb, which is governed by NO entry to the heme pocket, was reduced to half of the value obtained for HbA 0 . Moreover, Dex-BTC-Hb is only slightly more sensitive to oxidative reactions than HbA 0 , as shown by about 2-fold increase in autoxidation, and slightly higher H 2 O 2 reaction and heme degradation rates. Dextran-BTC-based modification of Hb produced an oxygen-carrying compound with increased oxygen release rates, decreased oxygen affinity and reduced nitric oxide scavenging, desirable properties for a viable blood substitute. However, the reduction in the allosteric function of this protein and the lack of apparent quaternary T->R transition may hinder its physiological role as an oxygen transporter.