Surveillance for influenza was carried out during a B/Hong Kong epidemic, Jan.-Feb.,1977, to determine clinical, epidemiologic, viral and immunologic characteristics of illness in infants. Fifty-nine (32%) of 186 infants and young children followed in an experimental vaccine clinic undergoing initial exposure to influenza B had viral or serologic evidence of infection. Thirty-one, 53%,of the 59 infected children were seen by a physician during the 4-week epidemic. Infants with virally proven influenza B infection were more highly febrile, mean 1028F, than either 23 school age children, mean 1003F, or 75 college students, mean 1007F, studied simultaneously . Signs of lower respiratory tract illness and otitis each occured in 19% of infants. Lower respiratory tract signs occured in 5% of school age and college patients Influenza B infections in infants were clinically indistinguishable from recently documented influenza A infections in the same population. HAI seroconversion is uniformly seen after primary influenza A infection. However, after influenza B infection the HAI response was inconstant, 8 of 16 seroconverted, and of low titer, mean post-epidemic titer 8.4. Eleven children had received inactivated influenza B vaccine in 1974. Eight of 11 had measurable pre-epidemic antibody. Six of 8 children with titers of ≥ 16 were infected. Five of the 6 had symptomatic respiratory illness. None of 3 children with titers ≥ 32 were infected. The HAI response after natural infection was enhanced in vaccinees, mean post epidemic titer 84.