342 C. E. SCHWERDT 



coefficient of variation of virus to reference particle ratios is found to be 

 about 15 % when counts are computed from 5 micrographs. 



This procedure permits counts to be made of particles suspended in 

 sohitions of nonvolatile salts without using the more elaborate equipment 

 required in the sedimentation method. The principal source of error in this as 

 well as the spraying technique for the determination of absolute titers of 

 particles is the estimation of reference particle concentration which, in turn, 

 is calcidated from the weight of a dried sample of this suspension and the mass 

 of a single latex sphere as computed from measurements of density and 

 diameter. 



4. Adsorption 



Another method for the quantitative assay of virus particles by electron 

 microscopy depends upon the specific adsorption of the particles to susceptible 

 cells (e.g., phage particles to susceptible bacterial cells) or to hemolyzed red 

 blood cells in the case of the myxoviruses (mumps-Newcastle disease- 

 influenza group). From micrographs of shadowed preparations of such 

 adsorption mixtures the average number of virus particles per cell can be 

 counted and the absolute virus particle concentration computed from the 

 known cell concentration. Luria et al. (1943) counted phage particles in this 

 way. Various members of the myxovirus group were similarly counted by 

 Dawson and Elford (1949) after adsorption to hemolyzed chick red blood 

 cells. Donald and Isaacs (1954a) also made particle counts of adsorbed 

 influenza virus, as well as of various other myxoviruses (Isaacs and Donald, 

 1955) and found them to compare favorably with parallel counts by the spray 

 droplet technique. Investigations employing this method of enumerating 

 hemagglutinating virus particles have been reviewed by Isaacs (1957). 



It must be remembered that the adsorption technique only accounts for 

 those particles which have adsorbed and teUs us nothing about the numbers 

 which failed to attach. Furthermore, the assumption must be made that the 

 adsorbed particles are randomly distributed over the entire surface of the cell 

 and, thus, from average counts per unit area, estimates are made of the 

 number of adsorbed particles on the underside of the cell and also overlying 

 the nucleus. In general, however, the assumptions and allowances appear to 

 be justified, since the results correspond to those obtained by the more 

 direct spray droplet technique (Isaacs and Donald, 1955). 



B. Indirect Methods 



1. Based on Physical Properties 



The earliest attempts to determine the particle concentration of a virus 

 suspension were based upon an estimate of the mass of a dehydrated virus 



