The pattern of growth of meningopneumonitis virus in vitro seemed to be similar to that occurring in ovo and thus the initial stages of development, the adsorption and the latent periods, were investigated by the use of tissue culture procedures.

The initial increment of infectivity in allantoic membrane suspensions following virus inoculation in ovo was due to prolonged adsorption of virus and not to immediate virus reproduction. The length of the adsorption period varied with the virus dilution employed.

The reduction of virus titer in allantoic membrane suspensions subsequent to adsorption was due to a change of infectious virus to a non-infectious form and this seemed to be a part of the normal developmental cycle of the virus.

The possible causes for both prolonged virus adsorption and for the subsequent development of a non-infectious form are discussed.

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