A sensitive electron microscopic method has been developed to determine the antigenic relationship within the picornavirus group. Negatively stained mixtures of antigens (poliovirus type 1 and 3, echovirus type 9, and coxsackievirus type A9) and antibodies (poliovirus type 3 and echovirus type 9 antisera) were examined under the electron microscope. The results showed clumping of homologous and heterologous viruses with both antisera, which was considered as evidence of cross-reaction. The controls showed no clumping or attachment of antibody molecules. The significance of clumping was tested by treating adenovirus type 7 with the two test antisera.