Analysis of fast-growing reassortants (AWM viruses) of influenza A virus produced by mixed infection with a fast-growing WSN strain and a slowly growing Aichi strain indicated that the M gene plays a role in the regulation of virus growth rate at an early step of infection (J. Yasuda, T. Toyoda, M. Nakayama, and A. Ishihama, Arch. Virol. 133:283-294, 1993). To determine which of the two M gene products, M1 or M2, is responsible for the growth rate control, one recombinant WSN virus (CWA) clone possessing a chimeric M gene (WSN M1-Aichi M2) was generated by using an improved reverse genetics and transfection system. The recombinant CWA virus retained the phenotype of both large plaque formation and early onset of virus growth. This indicates that the WSN M1 protein is responsible for rapid virus growth.