In order to further establish the safety of RA 27/3 vaccine and the quality of its antigenic response, both subcutaneous and intranasal administration of vaccine were studied. A contact study carried out in 70 families proved RA 27/3 vaccine to be noncontagious. The antibody response (hemagglutination-inhibition test) following vaccination, checked in 387 seronegative individuals, was only slightly inferior to that in individuals with preexisting naturally acquired antibodies. In children the failure rate was 7% when vaccine was administered intranasally and 0% when given subcutaneously. In adults, who were vaccinated only subcutaneously, 4% did not develop seroconversion. After three years, decline of the level of vaccine-induced antibodies was slightly faster than that of naturally immune individuals. In no instance was there loss of a significant titer. Negligible side reactions consisted in a few cases of slight indolent swellings of the nuchal glands. Rarely was a faint transient rash noted. No individual developed arthralgia or arthritis.