A new method to measure the neutralization of interferon by antibody has been described. The assay is based on the enumeration of infectible cells in smears of L cell cultures which after treatment with interferon-antibody mixtures and superinfection with challenge virus are stained with antiviral fluorescent γ-globulin. Titrations of interferons elicited by viral and nonviral stimuli in tissue cultures as well as in mice, and belonging to different molecular weight categories, revealed

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... comparable dose-response curves. Similarly, these mouse interferons obtained by various methods were neutralized to the same degree by serial dilutions of an antiserum prepared against Newcastle disease virus-induced L cell interferon. It is concluded that the classes of interferon studied do not exhibit significant antigenic dissimilarities detectable by this study.