Colonies of cells were obtained from human fetal lung tissue and exposed to recently isolated respiratory viruses. There was a considerable variation in the number of rounded cells found in different colonies exposed to rhinovirus types 2 and 9 (RV2 and 9), human coronavirus 229E (HCV), adenovirus type 3 (Ad3) and respiratory syncytial virus (RSV). Smaller colonies had more rounded cells than larger colonies. Clones were established from 9 out of 11 colonies. They varied in their rate of growth and the pattern formed on a plastic surface. They varied also in their virus susceptibility particularly to "difficult" rhinoviruses such as RV9 and SF1340. One cell clone (HL1/77 Clone 8), was highly susceptible to all these viruses. All cultures were more sensitive to RSV when maintained in F12K medium than in MEM, whereas there was no difference for rhinoviruses. Influenza A and B and parainfluenza 3 viruses sometimes produced cytopathic effect, and always produced haemadsorption, but unlike the previous strains could not be passed serially and presumably produced little infectious virus. All clones were rather insusceptible to Ad3; but the virus could be passed, whereas coxsackie virus B3 produced no CPE. Substantial yields of coronavirus and rhinovirus were obtained in gelatin sponge cultures. Two "very difficult" respiratory viruses which had just been adapted to tissue culture; namely, HS rhinovirus and JK coronavirus grew in 7 of 9 and in 6 of 9 clones respectively.