Summary: An antigenic analysis of 18 type 2 polio viruses has been made by means of cross-neutralization tests in tissue culture. Differing antigenic components entered into the constitution of the various strains making it possible to classify them into three major categories, namely, strains identical or closely related to the prototype Lansing virus, strains distinctly different from the Lansing virus, and intermediate strains. Differences in antigenic constitution of the type 2 polio viruses studied were observed in reciprocal CF tests using 1) the strains as viral antigens and studying their reaction with reference heterotypic sera, and 2) type 2 antisera and reference viral antigens of the three polio virus types and studying their reaction with antisera prepared against each of the type 2 strains under study. On the basis of these tests at least four of 15 type 2 polio viruses studied contained type 1 components in the composite antigenic structure.