A period of direct contact with a concentration of 20 mug/ml or less of either famotine (1-(p-chlorophenoxymethyl)-3,4-dihydroisoquinoline hydrochloride) or memotine (1-(p-methoxyphenoxymethyl)-3,4-dihydroisoquinoline hydrochloride) inactivates myxoviruses and paramyxoviruses. This activity has been confirmed in organ culture prepared from dog trachea and infected with an influenza A2 virus.Consistent protection of mice infected intranasally with influenza A/PR/8 has not been obtained. Other viruses causing respiratory disease in man, in particular some rhinoviruses, are susceptible during replication to the action of these compounds. Both compounds are of low toxicity and well tolerated by man.Trials in man have embraced challenge, prophylactic and therapeutic studies with both oral and local administration. Challenge studies with influenza A2 and B viruses have demonstrated that with some strains there is in the drug-treated groups a reduction in the incidence of evidence of infection, sometimes significantly, and also a reduction of clinical symptoms, the effect towards influenza B virus being the greater. Nasal instillation of famotine reduced the incidence of clinical symptoms only following challenge with influenza B virus and rhinovirus type 2. Oral prophylactic trials have been inconclusive, although the nasal instillation of famotine did reduce somewhat the incidence of symptoms in a small outbreak of influenza A2/Hong Kong/68. The number of therapeutic trials has been too few and too small to evaluate conclusively the role of these agents in therapy of acute respiratory illness.