Abstract
Mouse LM fibroblasts growing continuously in the absence of serum have an increased sensitivity to the cytotoxicity of poly(rI) X poly(rC) after interferon (IFN) exposure. This has allowed the isolation by an enrichment procedure of several independent and stable variant clones (IFN + I X C)R which are resistant to such a treatment. One of the resistant variants has been more extensively characterized as far as IFN action and IFN production are concerned. It behaves identically to the wild-type parent except for the spontaneous release of low amounts of IFN. The target of the mutation probably resides in a late step in the development of the cytotoxic response as revealed by microinjection techniques. The (IFN + I X C)R variants characterized here thus appear different from mutants in the IFN system isolated so far.