Two randomized, double-blind, placebo-controlled trials during early autumn of 1986 and 1987 evaluated the efficacy and tolerance of recombinant interferon-beta serine (rIFN-beta ser) nasal drops for prevention of natural rhinovirus colds. In 1986, 9 X 10(6) units of rIFN-beta ser (139 subjects) or placebo (157) were administered once daily except Sundays for 4 w. Rhinovirus colds occurred in 2.8% of rIFN-beta ser recipients and 6.0% of placebo recipients during the treatment period (52% reduction, P = .3). In 1987, 24 X 10(6) units of rIFN-beta ser (186) or placebo (197) were given daily for 25 consecutive days. Rhinovirus colds developed in 6.3% of rIFN-beta ser recipients and 5.3% of placebo recipients. In each study, illness frequency and number of days with subjective colds did not differ between the groups. Recipients of nasal drops of rIFN-beta ser at either dosage did not differ in tolerance from placebo recipients. The lack of both prophylactic efficacy and nasal toxicity are in contrast to prior observations with nasal sprays of rIFN-alpha 2b.