Immune electronmicroscopy (IEM) was used to identify human picornaviruses rapidly and to differentiate enteroviruses from rhinoviruses. Human sera, diluted 10- to 50-fold beyond the neutralisation endpoints for homologous virus, readily agglutinated C-type antigens of seven human picornaviruses. Human sera did not react by IEM with a control animal picornavirus. By IEM after acid treatment, differentiation of a human enterovirus from a human rhinovirus was possible. There was an excellent correlation between the results of IEM and immunodiffusion (ID) tests for the presence of antibody to human picornavirus group antigens. By ID, only one of 21 human sera reacted with one of six animal picornaviruses. Immune electronmicroscopy appears to be a sensitive and simple techinque for the detection of picornavirus C-type antigens, and may be useful for identifying viruses belonging to groups comprising many serotypes and sharing a common group antigen.