In a double-blind evaluation of alpha 2-interferon as prophylaxis against naturally acquired respiratory infections, 120 adult members of 46 Australian families used 325 courses of intranasal spray during a six-month period, applying 5 million IU to the anterior nasal mucosa daily for seven days when respiratory symptoms developed in another member of the family. Used in this way, the alpha 2-interferon was well tolerated, and the rate of minor nasal bleeding (12 percent) did not increase with repeated courses. By comparison with the control group of 109 members of 49 families who used 319 seven-day courses of placebo spray, the users of alpha 2-interferon experienced 33 percent fewer days with nasal symptoms and 41 percent fewer episodes of "definite" respiratory illness. The users of alpha 2-interferon who were exposed to rhinovirus infections experienced 76 percent fewer days with symptoms and 86 percent fewer "definite" illnesses than their counterparts who used placebo. All of the observed clinical benefits, which suggested prevention of 6.8 "definite" respiratory illnesses per 100 courses of medication used, could be explained by a protective effect against illness associated with rhinoviruses that was not demonstrated for influenza A or B or coronavirus 229E.