Slenczka, W., G. Wolff and R. Siegert (Hygiene-lnstitut der Philipps-Universi-tdt, 355 Marburg/Lahn, West Germany). A critical study of monkey sera for the presence of antibody against the Marburg virus. Amer J Epidem 93: 496–505, 1971.—On the basis of serologic studies it has been assumed by others that natural infections with the Marburg virus were frequent in wild-living monkeys. We have shown that the antigens from organs of infected guinea pigs, which were used in these studies, frequently produce nonspecific reactions. Neither in complement fixation tests with an antigen prepared from persistently infected Vero cells, nor by means of the indirect immunofluorescence method, which is four times as sensitive, were we able to confirm the occurrence of Marburg virus specific complement-fixing antibody in wild-living monkeys. Thus the origin of the Marburg virus is still obscure.