A comparative study was performed as regards the behaviour of lymphocytic choriomeningitis virus (LCMV)--strains Fo-2 and 522--and of Tacaribe virus (strain Tr 11573) at different temperatures and pH values. All the three arenavirus strains had a high thermosensitivity and were rapidly inactivated in an acid medium. LCMV was more sensitive to UV irradiation than Tacaribe virus. The effect of sonication was also investigated.