Mouse immune-type interferon (type II), a lymphokine, caused the transfer of viral resistance from mouse L cells to human WISH cells. The interferon was incapable of protecting WISH cells in the absence of L cells. The transfer of viral resistance occurred with interferon preparations of various specific activities, and was in proportion to the interferon concentrations in the preparations. The transferred resistance had the characteristics of an interferon-induced antiviral state in that it was blocked by actinomycin D, effective against different types of viruses, and resulted from an action on the cell rather than on the virus. Mouse immune-type interferon was more efficient than virus-type (type I) at eliciting the transfer of protection. The transfer phenomenon may represent a mechanism for amplification of the interferon system as a host defense against viral infection. Further, it serves as a model for studying the mechanism of lymphokine-induced transfer of information between cells.