Counts of Virus Particles by Sedimentation on Agar and Electron Micrography. ∗

Summary

A method has been devised for counting virus particles in electron micrographs of the virus sedimented on an agar surface and removed quantitatively in a collodion pseudoreplica. With influenza virus A (PR8 strain) as test virus, the accuracy of the counts, under conditions practical for routine studies, was the same as that observed with the spray technic or by sedimentation of the agent on collodion. In common with the latter method, sedimentation on agar is applicable to materials of relatively low virus content and, since salt diffuses into the agar, to viruses harmed either by drying in the presence of salt or by salt-free water. The usefulness of this procedure is limited to the study of virus particles which can be recognized either because of characteristic morphology or of other properties that can be related quantitatively to virus behavior.