We have reviewed some of the functions of lymphokines, the soluble mediators of the cellular immune response, which are produced by lymphoid cells. We have particularily emphasized those lymphokines which have functions similar to those reported for interferon. Interferon is capable of both amplifying and depressing cell mediated immunological reactions in a dose dependent fashion. The other lymphokines, whose functions are implicit in their names, are identified by their activities in various assay systems and thus there may be fewer actual molecular species than the list of factors implies. Obviously, there is much work to be done at both the cellular and molecular levels in order to elucidate the mechanism of lymphokine function.