During a rubella outbreak, the effect of exposure on 33 susceptible persons, 22 vaccinees, and 66 others with immunity from earlier rubella was studied. Groups were examined daily and specimens were collected for virologic testing. Infection occurred in 22 of the susceptible persons; all tested were viremic, excreted copious amounts of virus, and had clinical rubella. None of the previously vaccinated or naturally immune individuals developed rubella. Antibody boosts indicative of subclinical reinfection occurred in five vaccinees and one naturally immune child; these six were not demonstrably viremic, and only two vaccinees excreted virus. Antibody level appeared to influence resistance to reinfection. However, any detectable antibody protected against disease, prevented viremia, and blocked or modified pharyngeal virus replication. It was concluded that rubella exposure of vaccinees with antibodies was unlikely to constitute a significant clinical or epidemiologic risk.