Recombinant alpha-2b interferon was evaluated in two controlled trials, each lasting for 2 months or more, with different dose levels and schedules of administration. The first study was conducted during a period of transmission of type A (H1N1) and type B influenza. At 2.5 x 10(6) IU per day, no effect on influenza infection could be detected, but there appeared to be an effect on rhinovirus isolation. During the subsequent autumn 1.7 x 10(6) IU per day was found to have only a minimal effect on rhinovirus infection (efficacy from 22 to 27%). Under similar circumstances the preceding year, but with a daily dose of 3.0 x 10(6) IU, efficacy had been 76%. Since there was no evidence of change in rhinovirus strains circulating or their interferon susceptibility, this represented a dose-response relationship. It was possible to evaluate side effects in the 1,200 individuals involved. A lower dose was associated with lower frequency of symptoms of blood-tinged mucus. Persons using a placebo spray had a higher frequency of this side effect than an observed control. Using the spray 5 days a week was no less likely to produce symptoms than everyday use. Once-daily use was less likely to produce side effects than twice-daily use. There was no indication of sensitization when interferon was used for two separate periods of 4 weeks.