The two genes CED-4 and CED-3 (the nematode homologue of interleukin-1beta converting enzyme, ICE) of Caenorhabditis elegans are implicated in the control of cell death, but the mechanism by which this occurs is unknown. Here we provide evidence that CED-3 and CED-4 both contain sequences with homology to a domain present in RAIDD and the prodomain of certain ICE-like proteases (caspases). This domain is known to establish an interaction between RAIDD and these caspases. Similarly, CED-4 was found to interact with CED-3. Thus, the activity of the death protease CED-3 appears to be controlled by CED-4 through a direct physical interaction.