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Twenty-eight adult volunteers were inoculated intranasally with para influenza 2 virus and eight developed illnesses; twenty-eight volunteers were given flanks' saline and one became ill. The illnesses occurred in volunteers given between 2 × 104 and 2 × 106 TCD 50 of virus. The most prominent symptoms were sore throat, nasal stuffiness and coryza; four of the eight volunteers had sufficient coryza to be regarded as having mild colds. Although only eight volunteers had clinical evidence of infection, twenty-four had laboratory evidence of infection as judged by virus re-isolation or antibody response. Neutralization, haemagglutination-inhibition and complement-fixation tests on paired sera showed that sixteen individuals had a fourfold or greater antibody response by one or more tests including five of the eight volunteers who were ill. Twenty volunteers, including seven who were ill, had reciprocal neutralizing antibody titres of eight or more before inoculation of virus so it seems that the illnesses were due to re infection in the presence of antibody. Evidence is presented which suggests that although illnesses occurred in the presence of antibody they were due to the para influenza 2 virus and not some other agent in the inoculum. The results of these experiments seem to fulifi the third of Koch's postulates for para-influenza 2 virus as a cause of respiratory disease in adults.
We wish to thank Dr P. A. J. Tyrreil for his advice during the course of this work and in the preparation of the manuscript. We also thank Dr K. V. Shah (The Johns Hopkins University, Baltimore, U.S.A.) for help in some of the early experiments and Miss B. Ridgwell for valuable technical assistance.