BACKGROUND: The Nedd2/Ich-1 protein belongs to a growing family of mammalian cysteine proteases similar to interleukin-1 beta converting enzyme (ICE). Because of their similarity to the Cacnorhabditis elegans cell death protein CED-3, the ICE-like proteins are thought to play a key role in the execution of apoptosis. The active form of ICE is a tetramer consisting of two heterodimers (p20 + p10)2 derived from the cleavage of the pro-enzyme. RESULTS: In the present communication we show that the p51 Nedd2 precursor (pro-Nedd2) is also cleaved into p20-like (p19) and p10-like (p12) subunits by extracts prepared from cultured cell lines. Extracts from apoptotic NIH-3T3 cells but not normal growing NIH-3T3 cells also contained pro-Nedd2 cleaving activity. The processing of pro-Nedd2 by cell extracts was inhibited by characteristic inhibitors of ICE-like proteases. Additionally we show that pro-Nedd2 (p51) can be processed in vitro by active CPP32 and ICE, and to a lesser extent by Mch2 and Nedd2. Granzyme B, a serine protease required for cytotoxic T lymphocyte (CTL) mediated killing of target cells, also cleaved pro-Nedd2 to p19 + p12 subunits. CONCLUSIONS: Our observations suggest that Nedd2 activation requires cleavage by one or more ICE-like proteases that lie upstream in the proteolytic cascade. Cleavage of pro-Nedd2 by granzyme B indicates that Nedd2 may be one of the downstream effectors in the CTL-mediated killing of target cells.