For the purpose to evalute the properties of rinderpest virus, scanning and transmission electron microscopic studies were conducted with MDCK cells inoculated with rinderpest virus by observing changes in the surface and intracellular structures in these cells. The results may be briefly presented as follows. (1) It is possible to distinguish readily multinucleated giant cells produced by rinderpest virus from normal cells because of their flat shape. Moreover, the microvilli on these multinucleated giant cell surface are markedly less than those on normal cells. Yet microvilli do not necessarily disappear completely. (2) Even when infected by the virus the cell that does not transform into a multinucleated giant cell does not show so much change exteriorly as compared with the normal cell except a little shrinkage. (3) As for observation on intracellular changes it was not possible to clarify any morphological correlation between intranuclear inclusion body and cytoplasmic inclusion body. (4) It was confirmed that virus particles are budding out from the infected cell surface as well as from microvilli surrounding the cell. (5) Particles of 150-340 nm in size arranged in a chain formation have been identified to be rinderpest virus by comparing the negatively stained picture of virus particles specimen isolated from the infected cells with purified virus particles negatively stained specimen in the same field by scanning microscopy.(6) Mature virus particles reveal the morphology that is irregular in shape and size by scanning electron microscopy, ultrathin sections, and negatively stained specimen, whose size ranged about 150-340 nm. In addition, the negatively stained picture shows the presence of spikes about 8 nm in size on the virus surface, and these can be observed nucleocapsids of about 17 nm in diameter with the lumen of 6 nm. And there can be seen also filamentous virus-like particles about 1 μm in length. However, by the scanning electron microscopy on the present stage it is impossible to detect such ultrastructures. (7) It has been demonstrated that the cell membrane where virus particles bud out tends to be electron dense membrane. (8) The halo observable around the inclusion body in hematoxylin-eosin stained specimen is an artifact.