An anticomplementary activity of native dextrans of various structure was studied among the nine dextrans studied, eight polymers exhibited the anticomplementary activity of variable magnitude; only one dextran, containing 95% of 1--6 links and 2% of 1--2 links was unactive. The highest activity was exhibited by dextrans with high content of 1--2 links; lower activity was observed in dextrans, containing high amount of 1--3 links. The anticomplementary activity was not found in partially hydrolyzed dextrans, containing 90% and more links of the 1--6 type, with molecular weight of 2-10(6) daltons and lower. Polyaldehydes, obtained from these dextrans by oxidation with iodic acid, acquired the anticomplementary activity, comparable with the activity of native dextrans.