AbstractTwo subpopulations of antigenically different HswlNl influenza viruses, cloned from ‘swine’ New Jersey virus 1976, were individually inoculated into antibody‐free volunteers. One clone contained a haemagglutinin so far seen only in swine viruses prevalent in 1971 and after, the other a haemagglutinin of earlier strains dating back to at least 1957. Each of the viruses was infectious for man and intermediate in human virulence between a wild human virus and swine pathogens of 1966 and 1967, which had earlier been tested in man. Antigenically comparable clones segregated from viruses recovered in Wisconsin from a pig and a human contact, respectively, were also infectious for man; however, they were less virulent than their New Jersey counterparts. Differences between the Wisconsin clones themselves were small, but there was an indication of a relationship between human passage and human virulence. There was no evidence of inherently greater human virulence in the newer Hsw 1N1 serotype as compared with the earlier serotype. Hence, recent detection of swine influenza‐like viruses in man is unlikely to be the consequence of a host‐range mutation concurrent with an antigenic mutation.We believe that the Hsw1N1 viruses recovered from the human influenza outbreak at Fort Dix, New Jersey, and from recent single human infections were wholly derived from enzootic swine viruses that underwent limited human adaptation through man‐to