Data from the plaque assays, from the virus yield studies and from the athymic animal experiments all suggest that tumor cells are susceptible to NDV replication and virus-mediated cytolysis, while normal cells are not. The selective sensitivity of tumor cells to NDV may be due to the unique presence of a viral membrane receptor or to differential uptake, replication, or release of the virus. It
is unlikely that the effects are due solely to selective infection of rapidly growing cells, since NDV had little effect on rapidly dividing leukemia cells (Lorene and Peeples, unpublished data). Nor is it likely that the cell of origin determines the sensitivity to NDV, since tumor lies representing three diverse classes (epithelial, mes- enchymal, and neural) were examined and were all susceptible to the effects of NDV.