Cell lines of primate origin carry membrane receptors which are specific for echoviruses (EV). The present report describes isolation and characterization of a monoclonal antibody (Mab 143) reacting with the membrane of KB cells. The Mab was selected for its protection of different cell lines from primate origin against the CPE of EV-11. This protection was found to extend to most EV serotypes and to coxsackievirus A9, while the replication of several other picornaviruses was not affected. The fluorecein isothiocyanate labelled Mab did not react with cell lines from bovine, canine, or rabbit origin, but bound specifically to the cell lines from human or simian origin. These results suggest that a unique receptor site is used by most EV serotypes for binding onto and penetration into susceptible cells.