The rate of development of interferon-induced virus resistance in a mixture of two human cell types (U and WISH) is determined by the cell type (WISH) in the mixture which responds first. This phenomenon has been shown with two types of interferon assay procedure, and with both vesicular stomatitis virus and Sindbis virus. The transfer of virus resistance from one human cell (WISH) to another (U) (homospecific transfer) is much more efficient than the transfer from mouse L cells to WISH cells (heterospecific transfer), as shown by a much lower ratio of donor to recipient cells required for maximum transfer as well as a more rapid transfer. Thus, virus protection afforded by the interferon system is amplified more efficiently in mixtures of different human cells than in mixtures of mouse and human cells. These results suggest that, in a mixed population of cells such as occurs in vivo, more slowly responding cells might be influenced by cells which respond more rapidly to interferon. A defensive role is suggested for this mechanism which amplifies protection due to interferon.