We presented a cell kill pharmacodynamic model for cell cycle phase-specific antitumor agents as well as that for cell cycle phase-nonspecific agents. The former was based upon assumptions that a cell population could be divided into a drug-sensitive population having sensitive (or specific) cell cycle phase(s) and a drug-resistant population in resistant cell cycle phases. This model revealed that cell killing action of antitumor agents with sufficient cell cycle phase specificity was time dependent. This time dependence was actually observed with cytosine arabinoside by colony-forming inhibition assays using Chinese hamster V79 cells. On the other hand, the present analysis indicated that cell killing effects of drugs lacking cell cycle phase specificity are dependent on C x T (concentration-time product). We analyzed cell kill kinetics of cisplatin and showed C x T dependence of cell killing action of the drug using human colon cancer WiDr cells. These results indicate that cell killing action of antitumor agents can be kinetically analyzed in terms of cell cycle phase specificity.