Mass immunization with live measles vaccine has been carried out in rural Iran since 1968. Two strains have been used: primary baby calf kidney cell-adapted Sugiyama strain and human diploid cell-adapted AIK strain. More than 94% of the susceptible children experimentally vaccinated with either of the two vaccines have shown seroconversion. Mass immunization in rural regions has covered about 80% of susceptible children. It is now recommended that the live vaccine be 'administered twice: the first dose at six to nine months and the second at 12–15 months of age. In all of 100 cases of subacute sclerosing panencephalitis (SSPE) observed in the Tehran region between 1977 and 1982, the patient had a history of measles infection in childhood; there was no indication that SSPE developed after vaccination. Most of the patients with SSPE have had a high titer of antibody to measles virus in serum and cerobrospinal fluid, and antibody has commonly been demonstrated in saliva as well.