Functional analogues of vertebrate inflammatory cytokines have been described in a variety of invertebrates1. The analogy is based mainly on the crossreactivity of antibodies elicited against vertebrate cytokines, the sensitivity of invertebrate immunocytes to the action of vertebrate cytokines, and the responsiveness of vertebrate immune cells to invertebrate factors. But without knowing the amino-acid or gene sequences of the putative invertebrate cytokine analogues, it has not been possible to demonstrate unequivocally a phylogenetic relationship between vertebrate cytokines and their invertebrate functional analogues. Here we show that, although a defence molecule from the earthworm Eisenia foetida (Oligochaeta, Annelida) and the mammalian tumour-necrosis factor TNF-α perform similar functions, they emerged independently during evolution.