Abstract
Rat interferon of relatively high specific activity (about 106 units/mg protein) was produced in embryonic rat cells treated with Newcastle disease virus at a high m.o.i. The cells were cultured in serum-free medium and the interferon was precipitated and concentrated with 0.02 M-zinc acetate or with ammonium sulphate at 85% saturation. With both methods the increase in interferon activity was greater than the concentration factor. The rat interferon activity was stable on treatment with 0.15 M-perchloric acid or three cycles of freezing and thawing, but incubation at 37 °C for 1 h resulted in a 50% loss in activity. It had no cross activity in human or mouse cells. The sensitivity of different types of rat cells for rat interferon differed widely and was dependent on the challenge virus. Human interferons had no detectable antiviral activity on rat cells and did not block the activity of rat interferon.