Of 129 strains of the Hong Kong variant of influenza A2 virus isolated from 176 respiratory infections, 105 (81.4%) were isolated in tissue culture, the remainder being detected only by the inoculation of fertile hen\'s eggs. Organ cultures of chick embryo trachea were at least as sensitive as monkey kidney tissue culture for the isolation of this virus but organ cultures of human embryonic ciliated epithelium were much less efficient. The reverse was true for strains of influenza B virus studied in 1970 and 1971 when eggs and chick embryo tracheal organ culture were of little value and organ cultures of human embryonic ciliated epithelium and monkey kidney tissue cultures were the systems of choice.