Based on a pharmacokinetic model proposed by Jusko, which assumes that the cell killing action of cell cycle phase-non-specific agents occurs as a bimolecular reaction depending on drug concentration and cell density, we derived a cell kill kinetic equation for these drugs, including the decomposition constant in culture medium. This equation revealed that the cell killing activity of these drugs depends on the value of concentration x exposure time or the area under the drug concentration--time curve (AUC). It was also clarified that the curves for concentration--exposure time necessary for 90% cell kill on a log scale simulated on the basis of the equation differ according as whether drugs are stable or unstable in the culture medium, being expected to be linear with a slope of -1 in the former case, and to take the form of an asymptotic curve in the latter. For three cell cycle phase-non-specific agents, mitomycin C (MMC), 1-(4-amino-2-methylpyrimidine-5-yl)-methyl-3-(2-chloroethyl)3-nitrosoure a hydro-chloride (ACNU), and nitrogen mustard (HN2), we assessed the concentrations necessary for 90% cell kill (IC90) with various exposure times and the degradation rate constants under the culture conditions used. MMC was quite stable during the incubation, while ACNU and HN2 were unstable. When IC90's and exposure times were plotted on the above-mentioned graph, a linear relationship with a slope of -1 was seen for MMC, while for ACNU and HN2 the anticipated asymptotic curves resulted. We also ascertained that the decomposition constants for ACNU and HN2 expected on the basis of these curves showed a good agreement with the corresponding experimentally observed values. These results indicate that the cell killing action of cell cycle phase-non-specific drugs can be well described by a pharmacodynamic model and equation employing their decomposition constants and are dependent on the concentration-time product.