The number of factors implicated in the regulation of cell proliferation and differentiation is already considerable and more are continually being identified. This book concentrates on tumor necrosis factor (cachectin) and lymphotoxin, but includes observations of their interactions with other cytokines, especially the interferons and interleukins. TNF can be either cytostatic or cytotoxic to cultured cell lines, and a variety of mechanisms are proposed, ranging from DNA fragmentation to activation of phospholiopases. TNF is also reported to stimulate the growth of normal fibroblasts in vivo. The in vivo actions of TNF described here include haemorrhagic necrosis of tumors, cachexia, effects on the vasculature, and antiparasitic and antiviral activities. Also presented is some interesting data from the first stages of clinical trials of TNF as an anticancer agent.