1. It is likely that certain tailed and filamentous particles seen on electron microscope examination of partially purified saline suspensions of Newcastle virus are the individual virus particles because:
(a) They have a highly characteristic shape not seen in other virus preparations.
(b) They are present whenever the virus is present in high concentration.
(c) Their size agrees with the size of the virus as calculated from light scattering and centrifuge data.
(d) They are agglutinated by specific antisera.
(e) Infection may be produced in the embryo by relatively few of these particles.
2. It is possible that these filamentous forms have been derived from spherical forms without loss of activity because:
(a) Such filamentous forms are not found in the original allantoic fluid when this contains a comparable amount of virus.
(b) Filamentous forms appeared in the original allantoic fluid when it was dialyzed against saline solution.
(c) Filamentous forms were produced at certain hydrogen ion concentrations but not at others, in solutions maintaining the same infectivity for the embryo.
(d) Spherical forms were obtained by suspending the partially purified virus in water instead of saline. In this the virus remained moderately stable.
(e) These round forms could apparently be converted into tailed and filamentous forms by the addition of saline, again without loss of activity.
(f) This "conversion" could be inhibited by partial inactivation of the water suspension of virus.