Measles virus has been disintegrated by treatment with ether and Tween 80. This destroys the infectivity, and physically disintegrates the particle, with the release of an inner component structurally like the nucleoprotein of Newcastle disease virus (NDV), and another structure similar to the haemagglutinin (HA) of NDV. The preparation after ether-tween treatment has an enhanced HA activity. The two components are separable in a CsCl density gradient. The inner component is probably a nucleoprotein. The HA component could be adsorbed by monkey erythrocytes but not eluted from them. The action on it of sodium metaperiodate suggests that a carbohydrate may be involved. The ether-tween preparation could be used as an antigen in the CF-reaction with antisera to rinderpest and distemper. It could also be used in the HA-inhibition test as a sensitive indicator with antisera to rinderpest and distemper, as well as to measles, giving higher titres in the HA-inhibition test than when the untreated virus was used. Injection of the ether-tween preparation stimulated the production of neutralizing, HA-inhibiting and complement-fixing antibodies in the rabbit.