p53 is able to recognize and bind sites of DNA damage and, in some way, damage to cellular DNA activates a p53 response leading to G1 arrest or apoptosis. We have previously shown that 'damaged DNA' induces N-terminal cleavage of p53 to generate p40(DeltaN) and p35 (core) protein products. We now show that the p35 product has protease activity and is able to cleave between residues 23 and 24 of full-length p53 to generate a novel product, p50(DeltaN23). This activity was inhibited by bestatin, an aminopeptidase inhibitor. Residues 23 and 24 lie within the mdm-2 binding domain of p53 and the possibility that p50(DeltaN23) may be resistant to feedback regulation by mdm-2 is discussed. Unexpectedly, interaction with ssDNA induced two further cleavage products of p53, generated by C-terminal cleavage and designated p50(DeltaC) and p40(DeltaC). In vivo generation of a C-terminal cleavage product of endogenous p53 similar in size to p50(DeltaC) correlated with up-regulation of p21 expression in ML-1 cells exposed to either adriamycin or cisplatin. The possible significance of the various p53 cleavage products in relation to the cellular response to DNA damage is discussed.