Insufficient immunization is the major factor accounting for the failure to eradicate measles in the US; over 70,000 measles cases were reported in 1971, predominantly in previously vaccinated children. 4 reasons have been suggested for these apparent vaccine failures: 1) improper storage or handling of vaccine, leading to inactivation of the live virus; 2) inhibition of the active response to live virus by maternally transmitted or artificially administered passive antibody; 3) a 5% seroconversion failure rate; and 4) development in some children of modified illness and a secondary type of antibody response. If a 10% failure rate is assumed for a given population, 7% of the vaccinees will be in the primary nonresponse category and 3% may be considered to have responded originaly but to have lost their immunity. All children vaccinated before the age of 1 year should be revaccinated. An additional 5-15% of children are in need of a booster; however, until a skin test for immunity is developed, it is not practical to test or revaccinate every child.