A compound, 1\'-methyl spiro (adamantane-2,3\'-pyrrolidine) maleate, chemically related to the antiviral drug amantadine, was tested for activity in vitro against a number of human respiratory viruses. By a variety of techniques, it was shown to be active against a wide range of human and animal influenza A viruses. The effect was, however, variable and ranged from high activity against two 1957 Asian strains to no observable activity against a 1971 strain. Like amantadine, the drug did not inhibit the growth of influenza B viruses. It was also inactive against a number of paramyxoviruses. Unlike amantadine, the drug did inhibit rhinoviruses, but to a lesser extent than myxoviruses. The coronavirus 229E was also sensitive to the action of the drug in vitro. Although an earlier trial in volunteers showed that, when given orally from 2 days before until 5 days after virus challenge, the drug was protective against infection with influenza A/Hong Kong/68 virus, a similar trial in volunteers challenged with rhinoviruses 2 and 9 revealed no useful activity against rhinoviruses in man.