Measles virus nucleocapsids were labelled with 3H-amino acids and 32P-orthlls (AV+). When analysed by SDS-PAGE, the two major capsid-associated polypeptides (P, mol. wt. 69,000, and NP, mol. wt. 60,000) were shown to be phosphorylated. Subsequent characterization of the phosphorylated polypeptides by acid hydrolysis and high voltage paper electrophoresis showed that serine and threonine were the major phosphorylated amino acid species. The similarities between the peptide phosphorylation patterns obtained in these studies and those reported earlier for the virus phosphoproteins produced in acute infections (Robbins & Bussell, 1979) indicate that major phosphorylative modifications of the capsid proteins are not involvedin measles virus persistence in AV3 cells.