We performed two studies to determine the dose-related tolerance and the efficacy of recombinant beta serine interferon (rIFN-beta ser) in experimental rhinovirus infection. In the tolerance study, 120 healthy adults received intranasal sprays of rIFN-beta ser or placebo daily for 25 d. No differences in nasal symptoms were found. Rhinoscopy detected more mucosal bleeding sites in high-dose (38%) versus low-dose (12.5%) or placebo (12.5%) recipients. There were more subepithelial lymphocytes in nasal biopsy specimens in the high-dose (54%) than the low-dose (17%) or placebo (17%) groups. In the efficacy study, 34 volunteers received daily nasal drops of IFN, beginning 36 h before challenge and for three days afterwards. The numbers of infections and days of shedding of virus were not reduced, but colds occurred less often in the high-dose (25%) than in the low-dose (55%) or placebo (64%) recipients. Nasal mucus weights were lower in the high-dose (mean +/- SD g per five days; 5 +/- 4) than placebo (31 +/- 37) recipients. Thus, rIFN-beta ser may have a more favorable therapeutic ratio than do previously tested alpha interferons.