During the last decade, various zinc salts have been used against the common cold syndrome, which is known to be initiated by respiratory viruses, particularly rhinoviruses. Using rhinovirus as the challenge virus, we investigated whether zinc salts (Zn) could potentiate the antiviral action of native human leukocyte interferon (HuIFN-alpha) and rHuIFN-gamma. We found that HuIFN-alpha was potentiated tenfold at rather low levels of IFN activity (0.6-0.8 U/ml), resulting in 100% protection. Zn alone gave only marginal protection, if any. In contrast to HuIFN-alpha, rHuIFN-gamma directly increased the cytopathic effect of rhinovirus at low levels (<2 U/ml) but protected the cells at higher IFN levels (5-20 U/ml). No potentiation was seen with Zn. HuIFN-beta protected against rhinovirus at the same doses as used with HuIFN-alpha, but in contrast to HuIFN-alpha, no potentiation was noted.