Virus replication, cytopathic effect and interferon production were measured in Namalva lymphoblastoid cells infected with measles viruses. Eight virus strains of different origin or passage history were compared. An inverse relationship seemed to exist between the abilities of strains to induce interferon and to replicate to high titres. Two representative strains were found to be highly sensitive to lymphoblastoid cell interferon, when tested in a line of monkey kidney cells (Vero). In contrast, Namalva cells were found to be highly insensitive to autologous interferon when challenged with measles or Semliki Forest virus (SFV).