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In the analysis of various properties of pathogenic bacteria most investigators have paid special attention to the morphology of the colonies, antigenic characteristics, and virulence. Similar methods are now used in virus research except that the morphological studies concern plaques, not colonies. The plaque under agar technique, introduced by Dulbecco (1952) into animal virology, has been used to distinguish western equine encephalomyelitis virus from Newcastle virus, vesicular stomatitis from foot-and-mouth disease virus (Sellers, 1955), and poliomyelitis from Coxsackie and ECHO viruses (Hsiung and Melnick, 1957). Plaque size varies from one strain to another within the same type of poliovirus (Dubes, 1956) and variations have been noted among mutants of the same strain (Dulbecco and Vogt, 1955). At the present time, the study of antigenic characteristics seems to be the most important tool for systematic virus classification. By this method 3 distinct immunologic types of poliomyelitis viruses (Bodian et al., 1949) and several distinct types of Coxsackie A and B, ECHO, adenovirus, etc., have so far been found. With regard to poliovirus, qualitative differences in antigenic constitution among heterotypic strains and quantitative differences among intratypic strains can be demonstrated, according to Wenner et al. (1956). The third property of importance in virus classification is virulence, particularly virulence for animals. Of special interest at present is the relative capacity of different viruses to infect cell cultures. (Deinhardt and Henle, 1957; Guerin and Guerin, 1957; Warren and Cutchins, 1957). Actually this property could be also interpreted as "differential virulence for tissue cultures," particularly if the virus propagation on each single tissue culture is analyzed quantitatively. The purpose of the present investigation is to study the propagation of polioviruses and adenoviruses in different types of human and animal cells, and to evaluate the tissue culture infectivity of these viruses by means of a new cytopathic index.

CYTOPATHIC INDEX: A NEW METHOD FOR THE QUANTITATIVE EVALUATION OF TISSUE CULTURE INFECTIVITY OF DIFFERENT VIRUS STRAINS