SummaryAn antigenic analysis was made of echovirus 30 and coxsackievirus B4 isolates by determining neutralization rate constants in neutralization kinetic tests. The seven echovirus 30 isolates included the prototype strain and six others isolated in Melbourne, Australia, between 1959 and 1982. Little antigenic heterogeneity was observed in contrast to the evidence of antigenic variation recorded in similar tests on seven coxsackievirus B4 isolates. These isolates also included the prototype strain, as well as six others isolated in Melbourne between 1958 and 1973. The results obtained with the coxsackievirus B4 isolates were in keeping with those observed particularly with the polioviruses and also coxsackievirus B4 by other workers. Those obtained with the echovirus 30 isolates were unexpected, as this virus is also a member of the enterovirus group.