Guinea pig cell cultures produced extremely low levels of interferon when induced with Newcastle disease virus (NDV), or double-stranded RNA, poly rI.poly rC. Priming guinea pig cells with mouse interferon or guinea pig serum interferon did not significantly enhance interferon production. However, intracardial injection of 10(9) plaque forming units of NDV into guinea pigs lead to interferon production to levels over 6000 units per ml of serum 6 hr after the inducer was administered. The antiviral agent in the serum had the characteristics of interferon. Guinea pig interferon showed low levels of activity on mouse and bovine cells, and no detectable activity on human or rabbit cells.