The cultured cell lines Yoshida ascites sarcoma, L-1210 mouse leukemia, OAT cell line derived from human lung cancer of the oat cell type, and P3HR-1 cell line derived from Burkitt's lymphoma have been used for the cell-killing kinetics study of anticancer agents and evaluation of the sensitivity of cells using the soft agar cloning assay method. It has been found that there are 2 types of actions: 1) Type I (cytocidal and concentration-dependent action). The dose survival curves of Type I agents fit the equation log S=log nminuskD (S, surviving fraction; D, concentration of agents; n and k are constant). The sensitivity of cells can be expressed by mean lethal dose 90% (MLD90=1/k). Four cell lines were compared on this basis, and some problems concerning the chemotherapy of human cancer are discussed. Alkylating agents and anticancer antibiotics belonged to this group.2) Type II (cytostatic and time-dependent action). The dose survival curves fit the Gompertz equation S=exp[(minusbeta/alpha)(1minus e-aD)] (beta, population reduction parameter; alpha, constant). The exposure survival curve is negative exponential, indicating that exposure time rather than concentration is the key fo effective cell killing of Type II agents. Difficulties in expression of sensitivity to Type II agents are discussed. Antimetabolites, Vinca alkaloids, and L-asparaginase belonged to this group. The cell-killing kinetics of anticancer agents and comparison of the sensitivity of cells may provide some indications not only of optimal dosage schedules but also of a new approach in screening systems for truly effective agents for human cancer.