THE protein p21 (WAF1, CIP1 or sdi1), induced by the tumour-suppressor protein p53, interacts with and inhibits two different targets essential for cell-cycle progression(1-8). One of these is the cyclin-Cdk family of kinases and the other is the essential DNA replication factor, proliferating-cell nuclear antigen (PCNA). We report here that separate domains of p21 are responsible for interacting with and inhibiting the two targets. An amino-terminal domain inhibits cyclin-Cdk kinases and a carboxy-terminal domain inhibits PCNA. Using these separated domains, we have determined that p21 inhibits different biological systems through different targets. The PCNA-binding domain is sufficient for inhibition of DNA replication based on sinian virus 40, whereas the Cdk2-binding domain is sufficient for inhibition of DNA replication based on Xenopus egg extract and for growth suppression in transformed human cells.