A temperature-sensitive mutant of Sendai virus with a lesion in the haemagglutinin-neuraminidase protein (HN) (ts 271) was used to study the effect of HN cell surface expression on the fate of infected BHK-21 cells. The total amount of HN was reduced in ts 271 virus-infected cells at the non-permissive temperature (38 degrees C) presumably due to degradation of the protein. At this temperature, neither HN nor a modified form of HN were found expressed at the surface of the infected cells. BHK-21 cells infected with ts 271 were nevertheless killed by the infection at 38 degrees C as well as at 30 degrees C. These results ruled out the hypothesis that the lack of HN cell surface expression could be the unique requirement allowing BHK cell survival.