Summary We have measured by radioimmunodiffusion (RID) the specific IgA and IgG polioantibody in the sera of 36 men and 36 women. We have found in 8 women the IgA polioantibody titer was equal to or greater than the IgG polioantibody titer. In 6 women, the IgA polioantibody was 50% of the IgG titer. No man was found to have an IgA polioantibody titer equal to 50% of his IgG polioantibody titer. In fact, 33 of the 36 men had an IgA polioantibody titer which was less than 20% of their IgG polioantibody titer. The other 3 men had an IgA polioantibody titer which was 25% of their IgG titer. These data show that the adult female population contains a significant number of women in whom the IgA polioantibody is equal to or greater than the IgG polioantibody. No such individuals were found among the adult male population. Thus we have demonstrated for the first time that there is a significant difference in the immunoglobulin pattern of the polioantibody in the serum of the adult male and the adult female.