Ten presumptive enterovirus isolates which could not be neutralized by type specific antisera to any prototype enterovirus were related to echovirus 22 using molecular, biologic and serologic methods. Viral protein fingerprinting and PCR first suggested that these strains were variants of echovirus 22. Three of the strains were echovirus 22 prime strains, i.e., antiserum made to the variant strain neutralized the variant and the prototype strain. The other strains were neutralized by antiserum to the prime strains. Unlike typical enteroviruses, echovirus 22 and 23 prototype viruses and 7 of the 10 variants were heat stable at 50 degrees C in H2O for 1 h.