The findings presented indicate that aureomycin could become associated with tissue of the chick embryo by both hematogenous distribution and direct adsorption.

Treatment of chick embryos infected with MP virus with 1 mg. of aureomycin by the allantoic route caused an inhibition of virus growth in the allantoic membrane.

The drug had no effect on "inert" virus, and appeared to have little effect on adsorption of virus to host tissue.

Complete inhibition of growth during the time interval corresponding to the first cycle of multiplication could be achieved only if the drug was administered within 6 to 8 hours after virus inoculation.

Partial inhibition of virus multiplication could be achieved even if the administration was delayed as late as 24 hours after infection.

In these experiments the chief role of the antibiotic appeared to be one of virustasis reflected in a prolongation of the latent period (non-infectious phase). The virus was able to resume its growth when a critical low level of the drug in the allantoic membrane was reached.

When infectivity titrations were carried out using various tissues and organs of treated and untreated embryos, it was found that no virus was detectable in the brains of treated embryos as late as 192 hours after inoculation of virus. This was in contrast with the findings in allantoic membranes and livers of such embryos; these organs showed virus at 120 and 144 hours, respectively. In untreated controls, virus appeared in membranes at 24 hours, in the liver at 48 hours, and in the brain at 72 hours.

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