Hemagglutination-inhibition antibodies were tested for in sera of 438 Egyptians aged six months to 48 years. The percentage of seropositivity increased with age; 70%–75% of the younger age group were seropositive, while almost 100% of the group aged 10–15 years were seropositive. Of a group of female students in secondary school, 81% were seropositive, and in housewives of childbearing age, 96%–98% were seropositive. In a group of medical staff including doctors, nurses, and laboratory assistants, 100% were seropositive. Inapparent infection and reinfection may be responsible for this variation with age and for antibody in those with no history of rubella. The serologic test is necessary for accurate detection of immunity to rubella, particularly in those at risk, i.e., women of childbearing age.