Febrile response of infants to increasing concentrations of standard, intact influenza virus or its ether isolated antigens is compared. Types A-2 and B virus were used. At maximum antigen doses ether treatment eliminated fever induction by the A-2 strain and reduced this property for the B strain. For either serotype, the standard virus vaccine contains at least as much pyrogen as did the corresponding isolated antigen vaccine at 8 times the antigenic mass.