The protective effect of crude rat interferon against infection by vesicular stomatitis virus (VSV) and herpes simplex virus (HSV) was assessed in two culture systems: the PC12 cell line and dissociated rat neurons derived from the superior cervical ganglion (SCG). Interferon was induced in rat embryo cells by inactivated Newcastle disease virus, and its effect was assessed by reduction of viral yields and prevention of viral cytopathology. Interferon protected PC12 cells, both in the presence and absence of nerve growth factor (NGF), against infection by both viruses, although at differing concentrations: protection against VSV was noted at approximately a 10-fold lower interferon concentration than that required to inhibit HSV infection. Dissociated SCG neurons were also protected, but higher interferon concentrations were required. These results demonstrate that the antiviral state can be established in neurons in response to interferon.