J. gen. Virol. (198o) 47, I7I-I82 171 Printed in Great Britain Interferon Production by Individual L Cells

Using a protected centre technique in which agarose prevents the diffusion of interferon from individual producing cells, we have shown that essentially every cell in a monolayer of mouse L cells can be induced to produce interferon by infection with Newcastle disease virus (NDV). The amount of interferon produced by individual cells appeared to be highly variable, even when cloned cells and viruses were used. U.v.-irradiated virus lost its capacity to induce interferon in L cells and to infect chick embryo fibroblasts at the same rate. A small proportion of cells (I × lO- ° to IO × lO-6) appeared to produce interferon constitutively. This fraction was increased threefold by u.v. irradiation of the cells, and up to Io-fold by exposing cells to the mutagen ethyl methane sulphonate