Abstract
Summary
A new method for the assay of chick interferon (IF) was devised. Serial dilutions of IF were inoculated into wells made in the agar overlay covering chick embryo cells, and after 1 day's incubation at 37° the wells received a constant dose of the u mutant of Sindbis virus. Three days later a large plaque whose size was almost independent of the challenge virus dose appeared around the control well. This plaque formation was inhibited by the IF pretreatment. Thus, one monolayer dish, 90 mm in diameter, sufficed to titrate each IF sample. The highest IF dilution which reduced the normalized plaque diameter (the plaque diameter minus the cup diameter) to less than half that of control was comparable to the end point determined by the ordinary 50% plaque reduction test. The standard deviation of end points was approximately ±2-fold. A 1000-fold fluctuation in the challenge virus dose did not influence the IF titer determined. When a small-plaque clone of Sindbis virus was used and DEAE-dextran incorporated in the agar overlay, the same result was obtained.
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