The serological and biological properties of the two antigenically identical lines of the prototype CHHE-29 strain of echovirus 17 were compared. One (Kansas line) had been passaged only in cultures of rhesus monkey kidney (RhMK) cells and the other (Baylor line) in cultures of RhMK and African green monkey kidney (GMK) cells. The former was less sensitive to antibody and insensitive to agar inhibitor, and grew well in cultures of cynomolgus monkey kidney (CyMK) cells but less rapidly in GMK cultures, while the latter was more sensitive to antibody and sensitive to agar inhibitor, and grew well in either CyMK or GMK cultures. Six local strains of virus were similar to the Kansas line and were unchanged by passages in CyMK cultures. The selection imposed by serial passages of these viruses through GMK cultures, however, suggested that virus of the Baylor line may be derived from a mutation or be a selected subpopulation of the original virus of the CHHE-29 strain.