A serological survey was carried out in the southeast of the Central African Republic in 1979 to investigate the incidence of Marburg virus in the human population. Serum samples were tested by the indirect immunofluorescence method on infected and inactivated African green monkey kidney Vero cells on slides prepared by the Center for Disease Control, Atlanta, Georgia, USA. Of 499 samples, 7 were positive. Two showed a high antibody titer of 1/64. Both patients were retested 10 mo. later and were again found positive at the same titer. These 2 antibody carriers had never had any hemorrhagic syndrome. This is the 1st evidence of non-apparent Marburg virus infection in man without any epidemic. These findings constitute a 1st approach towards the discovery of natural foci of this virus.