It has been well known that p21( WAFT / CIPl) as a cell cycle regulator inhibits an activation of cyclin-dependent kinases(CDKs) thereby blocking the G1/S transition. However, its function at G2/M checkpoint is still unclear. Therefore, we investigated whether p21 is involved in G2 phase in mouse fibroblast LPl-1 cells. After treatment with mitomycin C, DNA damaging agent, at early G2 phase, the cells were arrested at G2 phase. In addition, the activation of M-phase promoting factor(MPF) which is responsible for the M phase initiation was decreased to 50% of untreated group at late G2 phase. To investigate if p21 has a role in G2 arrest, we first obtained 351 nucleotides of p21 cDNA from NIH 3T3 cells using a RT-PCR. Using the p21 clone as a probe, mRNA levels of it were measured during the cell cycle. The mRNA levels of p21 were high until entry into S phase, showed low levels at mid S phase, increased to maximal levels at G2 phase and then rapidly decreased thereafter. In mitomycin C-treated groups, the mRNA levels of p21 at G2 phase were slightly increased compared with those of control. These results suggest that p21 is involved in mitomycin C-induced G2 arrest in LP1-1 cells and this arrest may be controlled by increasing mRNA level of p21.