The Copenhagen line of the PR8 strain of influenza A was shown to differ considerably from lines maintained in other laboratories. Furthermore, its antigenic composition was found to have changed after numerous passages in eggs. Although the strain had remained unaltered for at least 107 egg passages, the 130th passage revealed the coexistence of two components one of which resembled the virus of earlier egg passages and the other that present subsequently. The latter virus did not resemble any of the influenza A strains which had been carried simultaneously in the laboratory. Thus, substitution or contamination and subsequent replacement by a related virus could be excluded with reasonable certainty. Although a genetic recombination may have resulted between a contaminating and the original strains, it is held more probable that at some time between the 107th and 130th egg passages a mutant developed which presumably outgrew and replaced the parent virus in succeeding passages.