serum treatment in ooo died without evidence of anomalous development, despite the fact that virus could be recovered from their tissues. A possible explanation for this phenomenon lies in the knowledge that the chick embryo is susceptible to lethal infection with this virus for at least the first 2 weeks of development, while the period of susceptibility to the teratogenic effects of the virus is restricted to about the first 48 hours of embryonic development.Conciusions. Specific immune serum will protect the 48-hour chick embryo from the teratogenic and lethal effects produced by infection with influenza-.\ virus. Inasmuch as antiserum can neutralize these effects completely in vitro, and to a lesser extent in OVO, this study provides serological evidence to implicate the virus as the cause of the syndrome described.