Immunoglobulin M antibodies can be measured by indirect immunofluorescence in sera of patients suffering from Lassa fever or Marburg virus disease 4-7 days after onset of illness. Titres reach a peak 1-2 weeks later. These antibodies disappear, or titres decrease considerably, 1-2 months after onset of illness. Antiviral IgG antibodies can be detected at the same time as, or a little later than, IgM antibodies, but they persist much longer. None of the three patients discussed in this paper who died of Lassa fever developed IgG antibodies and only one developed IgM antibodies.