Both killed unadjuvanted ferret and egg grown A/Moscow/1019/65 (H2N2) influenza virus failed to immunize ferrets against challenge with homologous virus: the preparations were given in 2 doses, 2 wk apart, distributed intranasally, intramuscularly and intraperitoneally. However, small doses (<2 HA units) of both preparations induced immunity in ferrets previously 'primed' with a live heterologous virus (A/FM/1/47 (H1N1) according to the method of Potter et al. (1973). Although no difference in immunizing activity was detected between ferret and egg grown virus, the former induced greater HI titres than the latter. There was no correlation between HI titres in serum and protection to challenge: in fact, some protection seemed to be afforded by the 'priming' virus in the absence of HI antibody to the challenge virus. These results are discussed in relation to the possibility that a previously unrecognized antigen different from hemagglutinin and neuraminidase may contribute to immunity to influenza.