Summary The effect of dextran sulfate on the poliovirus strain L Sc 2ab increases up to concentrations of ∼ 100 μg/ml. The effect of low-molecular-weight dextran sulfate is slight, but increases rapidly when the molecular weight rises above 100,000, probably reaching a steady state with molecular weights above 2,000,000. Dextran sulfate was shown to inhibit the reversible attachment of virus and had little effect on the subsequent steps of virus-cell interaction. It is also demonstrated that the inhibitory effect of dextran sulfate is probably due to absorption of sensitive virus to the polyanion.