Summary It is thought that influenza and related viruses enter susceptible cells, such as those of tissue cultures and the chorioallantois, by being adsorbed to the surface and then taken in by an active process termed ‘viropexis’. It has been suggested that this active process resembles phagocytosis (Fazekas de St Groth 1948). However, influenza viruses commonly invade the ciliated epithelium of the respiratory tract of the intact host, which is thought not to be actively phagocytic. Organ cultures of such epithelium are extremely susceptible to infection (Hoorn & Tyrrell, 1969). It was therefore of interest to use these to investigate the mechanism of entry of influenza viruses into ciliated epithelial cells; it was uncertain whether the primary target cells would be the ciliated or the mucus-secreting cells. As infection was so efficient, it seemed likely that the virus might ‘exploit’ in some way the sweeping action of the cilia and, rather than being moved on by their activity, might attach and then enter the cells directly or indirectly. Further studies on the entry of virus into non-ciliated cells appeared during this work and these will be discussed later.