After multiplication in the continuous human cell lines HeLa, KB, and Hep 2, echovirus 11 (prototype strain Gregory) loses its haemagglutinating (HA) activity but retains infectivity. After seven passages in Hela cells the HA activity was not restored. There was no correlation between the loss of HA activity and modification of thermosensitivity, plaque morphology, surface properties and antigenicity. HA- echovirus 11 particles did not show the properties of defective interfering (DI) particles as described for poliovirus.