Summary: The toxic effects of certain viruses have been modified by viral interference, prior injection of receptor destroying enzyme (RDE), heat inactivated RDE, or xerosin. Similarly, the neuropathic effect of egg-propagated Newcastle disease virus (NDV) for mice was also modified by these agents. One strain, Cal-NDV, which was neuropathic for mice only in low dilution was found to interfere in mice with the neuropathic effect of another strain, RO-NDV, which was neuropathic for mice in high dilution (10-5). Prior intracerebral injection of influenza B virus or RDE modified host response to both concentrated and dilute NDV. However, xerosin modified only the neuropathic effect of Cal-NDV and of concentrated RO-NDV, but failed to delay paralysis or death following inoculation of dilute RO-NDV. It is suggested that further study of the neuropathic effect of NDV for mice may lead to a better understanding of the possible relationships between viral toxicity, masked virus, and incomplete virus.