Inosiplex (Isoprinosine), the paracetamidobenzoic acid salt of inosine dimethylaminoisopropanol, has shown antiviral activity in cell culture and in animals. Controlled challenge studies using the drug in a prophylactic fashion, however, have been disappointing. In vitro studies, as well as uncontrolled clinical trials, have suggested that the drug might be more effective when used therapeutically. We therefore undertook to test inosiplex in a controlled, double-blind, therapeutic study of volunteers challenged with rhinovirus. Thirty-nine volunteers were randomly divided into groups receiving either inosiplex or placebo tablets. Drug or placebo was started either at the time of, or 48 hours after challenge with rhinovirus Type 21. Illness was assessed in terms of the classical common cold symptoms, and infection was also determined by viral isolation from daily nasal wash specimens and by serum antibody rises. Five of 19 volunteers in the inosiplex group became ill, whereas 14 of 20 in the placebo group were sick (p less than 0.01). There was no difference between the control and inosiplex groups in the number of volunteers from whom rhinovirus was isolated. There was, however, a reduction in the duration of virus shedding in the inosiplex group. Seroconversion was also slightly less common in the inosiplex group. Immunologic studies suggest that inosiplex stimulates the lymphocyte mitogenic response. The results suggest that inosiplex exerts significant therapeutic benefits in rhinovirus infection.