In the fall of 1983, 53 households were enrolled in a double-blind trial of alpha 2-interferon as an intranasal spray to prevent common colds. During the winter/spring of 1984, 26 households were infected with influenza type B, as shown by isolation of the virus (19 households) and/or significant antibody titer rises (seven households). Interferon did not prevent influenza B infection or modify resulting illness. Of 37 persons shedding virus, 12 were asymptomatic. All were older than age 12 years, and 10 did not respond with antibody by any of the five test methods employed (complement fixation, hemagglutination inhibition, enzyme-linked immunosorbent assay (ELISA), neutralization, and Western blot). In contrast, of 13 symptomatic persons shedding virus from whom sera were available, 11 had significant antibody titer rises. Infection rates were highest among teenagers, but also surprisingly high among the 11 persons observed who were aged 50 years or older, four of whom were infected. The case-to-case interval in household transmission varied between one and nine days. Longer intervals of one, two, and four months between infections among family members were also observed, suggesting repeated introductions. Neither virus isolation alone nor serologic tests was sufficient to estimate infection rates.