Fraction T2 from Tityus serrulatus venom produced a marked lengthening of action potentials recorded with the single sucrose-gap technique, a characteristic effect of alpha scorpion toxins. This effect was not reversed by thorough washing of the nerve. On the other hand, T2 fraction deactivated by complete iodination did not cause any alteration of the compound action potential, even if applied in concentration as high as 4000 times the half saturation dose of the unmodified T2 fraction. This high dose of deactivated T2 did not hinder the onset of the full effect of a single dose of T2 fraction applied subsequently. Polyclonal antibodies against native or against iodinated venom reverted the action of T2 fraction, restoring normal electrical response. We conclude that both types of antibodies may remove the effect of alpha toxins on sodium channel.