Measles virus was grown in HEp-2 cells and purified by sedimentation on potassium tartrate gradients. The virus formed a sharp band at a density of 1.23 (g/cm3). Treatment with sodium deoxycholate disrupted measles virus particles, releasing virus nucleocapsids which sedimented at approximately 280 S and were resistant to pancreatic ribonuclease. Equilibrium density sedimentation of nucleocapsids in CsCl gave a visible band at a density of 1.30 (g/cm3). The E280/E260 ratio of dissociated nucleocapsids was 0.88, consistent with an RNA content of 5.0%. RNA extracted from purified virus and nucleocapsid had a sedimentation coefficient of 52 to 54 S in sucrose density gradients and a mol. wt. of 6.4 × 106 when fractionated on polyacrylamide-agarose gels. The polypeptides of purified virus and nucleocapsid were separated by polyacrylamide gel electrophoresis. The intact virus particles contained six polypeptides of mol. wts. 75600, 69000, 60000, 53000, 51000 and 45700. The nucleocapsid contained a single polypeptide of mol. wt. 6 × 104.