Purified lymphoblastoid interferon (HuIFN-alpha) or placebo was self-administered intranasally by volunteers using a spray device three times daily for four and one-third days beginning one day before virus challenge. Each subject received a total dose of 35.1 Mu of interferon (IFN) administered in 13 equal doses of 2.7 Mu. Doses were administered in a volume of 0.2 ml (0.1 ml to each nostril). The first group received human rhinoviruses types 9 and 14. There were no significant colds in 19 volunteers receiving IFN and 7 in 23 volunteers receiving placebo (p less than 0.05). Serological responses and/or recovery of challenge virus were obtained in 14 (74%) recipients of IFN and in all 23 recipients of placebo (p less than 0.05). Mean daily and total clinical scores and mean daily and total nasal secretion weights were significantly greater in those receiving placebo than in those given IFN. The second group received influenza virus A/Eng/40/83. There were 4 significant illnesses in 13 volunteers receiving IFN and 10 in 17 volunteers receiving placebo (p greater than 0.05). Serological responses and/or recovery of challenge virus were obtained in 11 volunteers receiving IFN and 14 volunteers receiving placebo. Mean daily secretion weight and mean clinical scores were lower in those given IFN than in those given placebo - the differences were significant for clinical score on 2 days. The results suggest that IFN prophylaxis was less effective against influenza A than against rhinovirus.(ABSTRACT TRUNCATED AT 250 WORDS)