Abstract
We describe a simple, rapid and reproducible assay for defective- interfering Semliki Forest virus (DI SFV) which is based on the inhibition of synthesis of virus-specified RNAs in SFV-infected cells. Using the assay, we have been able to show that DI virus is generated by a single passage in baby hamster kidney (BHK) cells in an inoculum which contained no detectable DI virus and we have calculated the u.v. target size of the interfering activity.