Since the discovery of tumor necrosis factor (TNF) and lymphotoxin, many structurally related factors have been elucidated in mammals that are grouped together in the TNF Ligand Superfamily. These factors have profound pleiotropic roles involving inflammation, apoptosis, cell proliferation and stimulation of the immune system. TNF has only been isolated from mammals and fish. While two TNFs (alpha and beta) are present in mammals, it appears that only one form of TNF is found in fish that is more similar in structure and genomic organization to mammalian TNFalpha. TNF transcripts are expressed in both mammals and fish with similar kinetics, and the involvement of NF-kappaB in TNF expression in fish suggests that transcriptional regulation may be similar within vertebrates. Mammalian TNFalpha stimulates macrophage activity in fish and birds, and in both groups of vertebrates, macrophage-derived supernatants have been shown to contain TNF-like activities, suggesting the presence of bioactive native TNFalpha.