Strain-specific antisera against non-Sabin-like and Sabin-like poliovirus strains were prepared by cross-absorption of the antisera with the heterologous strain. Isolates from different parts of the world were differentiated intratypically as non-Sabin-like and Sabin-like strains with the aid of these sera in the immunodiffusion test and neutralization test. In comparison with other marker tests, intratypic differentiation can be performed more easily. Further, fewer intermediate strains were found and, in our opinion, the results are more reliable.