Previously it was shown that cocultivation of mouse L and human WISH or baby hamster kidney cells in the presence of mouse interferon resulted in decreased viral yield from both cell species. We now show that this phenomenon also occurs when rabbit kidney and human WISH cells, with their corresponding interferons, are cocultivated with human WISH and baby hamster kidney cells, respectively. This finding increases the number of donor cell types to three. The related finding that monkey VERO and chick embryo cells can be recipients of transferred resistance expands the number of heterologous recipient cell species capable of receiving transferred resistance to five. Not all cell types tested have been shown to function in this transfer system. The fact that VERO cells, which do not produce interferon, are capable of receiving transferred resistance is significant because it indicates that the mechanism of transfer does not involve production of interferon by the recipient cells. © 1978 Springer-Verlag.