A fibroblast-inhibiting agent, chloroquine, was administered against Basford cancer in combination with several anti-tumor drugs. 1) In combination with mitomycin C, cyclosphamide and hematoporphyllin-Hg, chloroquine showed more inhibitory effect on the growth of the tumor in comparison with single administration of these drugs. 2) The combination of chloroquine and prednisolone showed almost the same inhibitory effect on the growth of the tumor as single administration of either drug. 3) The combination of chloroquine and chromomycin A(3) was less effective than administration of chromomycin A(3) alone. 4) Histologically, the combination of chloroquine and mitomycin C induced an increase of necrotic areas, degeneration of cancer cells and inhibition of stromal connective tissue fromation in comparison with single administration of either drug. 5) From these results it is considered that a fibroblast-inhibiting agent, chloroquine, when used in combination with anti-tumor agents which inhibit DNA synthesis is more effective against malignant tumors.