TPA-treated HL-60 cells are mainly arrested in G1 by p21WAF1 accumulation. We investigate the downstream changes following such accumulation. Increased p21WAF1 is associated with CDK2 and CDK4. pRb is dephosphorylated in the presence of p21–CDK2/4 complexes, and the Rb–E2F1 complex increases after TPA treatment, whereas the Rb–HDAC1 complex decreases slightly. Our results suggest that increased p21WAF1 is associated with CDK2/4, and that these complexes induce pRb dephosphorylation. In turn, hypophosphorylated pRb are mainly complexed with E2F1, but HDAC1 appears not to be a key component in this process.