The prophylactic efficacy of low-dose intranasal recombinant leucocyte interferon alpha (rIFN-alpha A, Ro 22-8181) was investigated under field conditions in 147 families (n = 587 participants), randomized to receive placebo or rIFN-alpha A intranasally in daily doses of 1.5 or 0.3 X 10(6) IU. Treatment was started within 2 days after the appearance of an index case in the household and was continued for 5 days. Clinical data of the index case and of all members of the household were recorded for 10 days. In index cases and all ill contact persons nasal washes were collected for rhinovirus isolation and immunochemical detection of other respiratory viruses. The local tolerance of the intranasal rIFN-alpha A was excellent. Both doses of rIFN-alpha A failed to exert therapeutic effects on established common cold or to prevent the spread of common cold within families. Prophylactic treatment with 1.5 X 10(6) IU did however shorten the duration of the cold (median of 2 days vs. 4 in the placebo group, P = 0.01) and reduced the severity of any ensuing common cold (median total score of 10.5 vs. 30, P less than 0.001). No correlation was found between viral etiology (55% rhinoviruses vs. 13% other respiratory viruses, n = 122 nasal washes) and prophylactic efficacy or clinical severity.