A double antibody competitive binding radioimmunoassay (RIA) was developed as a tool for investigating the involvement of measles virus in persistent virus infections. The assay employs 125I-labelled measles virus nucleocapsids as the labelled antigen. Nucleocapsids were purified from cytoplasmic extracts of virus-infected Vero cells, treated with trypsin to prevent clumping and iodinated to a specific activity of about 15 microCi/microgram. Electrophoretic analysis of iodinated trypsin-treated nucleocapsids revealed only labelled polypeptide with a relative mol. wt. (Mr) 40 000, indicating that trypsin had cleaved the 60 000 mol. wt. native polypeptide to a 40 000 mol. wt. subunit. The assay could detect as little as 0.1 ng of nucleocapsid protein. Either native (Mr 60 000) or cleaved Mr 40 000) nucleocapsid polypeptide was detected in the assay. As much as 100 micrograms of protein from uninfected Vero cells did not react in the RIA. Another measure of specificity was the fact that 400 ng of purified nucleocapsid protein from simian virus 5, Newcastle disease virus or canine distemper virus did not react in the assay. In the RIA, nucleocapsid antigen of virus isolated from subacute sclerosing panencephalitis (SSPE) was indistinguishable from that of Edmonston strain measles virus, indicating antigenic homology between the 40 000 mol. wt. nucleocapsid polypeptide of the SSPE virus and that of measles virus.