Cytokines have various kinds of functions, including immunoregulation, host defense system, induction of inflammation and pathogenesis of many diseases. One of the characteristics of cytokines is the interaction among cytokines, and they further play an important role in the defense against tumor development, although its mechanism is highly complicated. In this paper, cytokines possessing antitumor activity, are described. These are 2 categories of antitumor cytokines, immunological and non-immunological ones. As immunological cytokines, interferons, interleukin 1, 2, 4, 5, 6, 8 and 12, tumor necrosis factor and lymphotoxin, have been investigated. On the other hand, tumor-degenerating factor, tumor regressing factor, glial maturation factor, glial growth inhibitory factor, neuroblastoma growth inhibitory factor, neonatal brain derived carcinostatic factor, chondromodulin and gliostatin have been characterized as non-immunological antitumor cytokines. These cytokines interact, and the complicated network of immunological and non-immunological antitumor cytokines is formed.