Four immunopotentiators (OK-432, PS-K, Bestatin, Levamisole) were used in combination with chemotherapy for treatment of Lewis lung carcinoma in BDF(1) mice. Tumor inoculation was done on Day 0 and chemotherapy with cyclophosphamide 200 mg/kg and ACNU 20 mg/kg followed on Day 7. Each immunopotentiator was administered on Day 2, 3, 4 or on Day 5, 6, 7 or on Day 7, 8, 9 or on Day 10, 11, 12. The effect of immunopotentiators depended on the combination timing of immunopotentiators and the chemotherapy, the combination timing being different for each immunopotentiator. A marked antitumor effect and the highest survival rate were observed when OK-432 was administered on Day 2, 3, 4 or on Day 10, 11, 12. PS-K did not show definite additional antitumor effects in every timing. The administration of Bestatin on Day 7, 8, 9 resulted in a good survival rate, but if it was given on Day 5, 6, 7, the survival rate was lower than that of control group. A good antitumor effect was observed when Levamisole was administered on Day 5, 6, 7 or on Day 7, 8, 9. At the same time, the influence of immunopotentiators on delayed hypersensitivity by picryl chloride was investigated. Also in this study, the time-dependent effect of immunopotentiators was observed. These results were similar to those of the study with the survival rate.