Marburg virus (MV) reproduction in organs, hematological and pathological changes were studied by virological and clinical methods, light and electron microscopy in guinea pigs respiratory challenged by the virus. Liver and spleen were most affected by MV, as in parenteral infection. The sequential involvement of cells in virus replication was also the same as in parenteral infection, with monocytoid-macrophagal cells infected first, followed by hepatocytes, spongiocytes, endotheliocytes and fibroblasts. Hemopoietic cells showed evidence of severe damage in respiratory infected guinea pigs. A distinguishing feature of the respiratory infection was close contact of leucocytes with MV infected cells. It is suggested that the entrapment and accumulation of MV in the lungs of respiratory infected guinea pigs makes possible the enfoldment leucocyte attack which does not, however, result in destruction of the infected cells.