COOPER1 placed measles virus in the deoxyvirus group. There is now considerable evidence which is contrary to this view. The appearance of acridine-orange stained inclusions in measles virus infected cells under ultraviolet illumination suggests the presence of a ribovirus2. This is in agreement with the observations of Atherton3. Waterson et al.4 reported similarity of measles virus to myxoviruses, under the electron microscope. Waterson5 further showed that the three serologically related viruses, measles, canine distemper and rinderpest, are structurally identical with Newcastle disease virus (NDV), representative of a myxovirus sub-group. Further support for the similarity of measles virus and NDV is that these viruses undergo identical morphological changes during ether treatment, and the ability of measles virus to cause hæmagglutination.