; GMK-AH 1 (Institute of Medical Microbiology, University of Kiel, Germany) in DMEM; Vero cells (ATCC No. CCL81) in DMEM; HRT-18 (human rectal tumour) cells (provided by Dr Herbst, Institute for Animal Hygiene and Infectious Diseases, University of Giessen, Germany) in DMEM; calf trachea cells in DMEM; HepG2 cells (supplied by ATCC cell HB 8065) in DMEM; and MA-104 in EMEM.