Anti-AIDS virus activity of synthetic and natural dextran sulfates was assayed in vitro by using a MT-4 cell line. These dextran sulfates with the number average molecular weight (Mn) of 1.8×104−10.7×104 showed potent anti-AIDS virus activity in the concentration as low as 3.3 or 10 μg ml−1, respectively. This high activity was almost the same as that of curdlan sulfates which was reported previously. The degree of sulfation per glucose residue in polymer backbone was 1.0–1.9 (the highest theoretical value is 3.0) which was calculated from the elemental analysis. The relationship between the structure and anti-AIDS virus activity is discussed based on high resolution NMR spectroscopy.