Enterovirus type 70 (EV70) agglutinated human 'O' erythrocytes at 4 degrees C as well as 22 degrees C, but visible agglutination was lost when warmed at 37 degrees C although the virus remained attached to the surface of the erythrocyte. The receptor sites for the virus were neuraminidase-sensitive. A direct involvement of sialic acid on the cell surface in virus-cell interaction was confirmed by the fact that the presence of fetuin or free N-acetylneuraminic acid inhibited the haemagglutinating activity of EV70. Similar numbers of virus particles were required for 1 haemagglutinating unit (HAU) of EV70 and 1 HAU of mengovirus, whereas 2.6-fold or more of virus particles of echovirus type 7 and type 11 gave the same activity. On the other hand, the number of receptor sites on the cell surface for EV70 was found to be sevenfold more than for mengovirus. Therefore, the erythrocyte receptor for EV70 is different from that for common enteroviruses and similar, though not identical, to the cardiovirus receptor. However, serological tests such as neutralization, complement fixation or haemagglutination inhibition did not reveal any common antigen between EV70 and cardiovirus.