A live, attenuated influenza B virus vaccine was evaluated in a group of students. The virus, cold-recombinant (CR) 7, was produced by recombining the attenuated cold-adapted parent, B/Ann Arbor/1/66, with a wild strain, B/Hong Kong/8/73. In ferrets, the wild strain produced histopathologic lesions in the lungs, whereas the CR strain and the attenuated parent did not. A total of 306 individuals was inoculated intranasally with either the CR virus or a placebo. After inoculation, only one symptom was significantly more common in the vaccinees than in the control subjects. That symptom, sore throat, occurred briefly in 26% of the vaccine recipients and in 10.5% of the placebo recipients. An outbreak of influenza B occurred three months after vaccination. When serologic infection rates in the two groups were compared, it was found that the vaccine had been significantly effective (P less than 0.01) in preventing infection.