The effects of simultaneous inoculation with two attenuated influenza A viruses was studied in ferrets and volunteers. Groups of ferrets were inoculated with an influenza A (H3N2) or (HlN I), virus or a combination of both viruses: the temperature response, serum and local antibody response, and the change in nasal wash protein concentration was determined. The results showed that both viruses were attenuated for ferrets, and that inoculation with both viruses together did not cause clinical reactions. Serological studies on paired serum samples obtained from ferrets showed that both viruses when given separately infected all the inoculated animals; however, dual infection resulted in all ferrets being infected with the influenza A (H3N2) virus strain, but this infection interfered with infection by the influenza A (HlNl) strain. Similar investigations were carried out in volunteers. Again, the clinical reactions and temperature response of volunteers to infection by one or other of the viruses showed both strains to be attenuated for man even when given together. In addition, no adverse clinical reactions were seen in volunteers inoculated with both viruses simultaneously. Serum antibody studies showed that infection by influenza A (HlN1) virus interfered with infection by the influenza A (H2N2) virus strain. These results show evidence of interference by influenza A viruses; however, the direction of interference was one-way, and differed for ferrets and for volunteers.