Mode of action, production and some properties of a viral growth-enhancing factor (enhancer) which was detected in allantoic fluids of eggs infected with parainfluenza type 1 virus were investigated. The factor markedly depressed interferon synthesis induced by u.v.-irradiated Newcastle disease virus in chick embryo cells, but did not inhibit the action of exogenous interferon. It had no effect on the multiplication of Newcastle disease virus in chick embryo cells. The depression of interferon synthesis by the factor was unlikely, therefore, to be due to an inhibition of the early steps of virus cell interaction. On the basis of its biological properties the factor should be termed ‘interferon depressor’. The appearance of the factor in allantoic fluids of eggs infected with parainfluenza type 1 virus could be determined by selectively destroying the interferon also contained in the materials. The activity of the factor reached a peak 24 hr after virus inoculation and remained stationary thereafter. Present data on the chemical nature of the factor are compatible with the view that it is a carbohydrate.


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