The effects of chloroquine on the growth and oxygen consumption of cultured fibroblasts were investigated. 1) Chloroquine diphosphate inhibited the growth of chick embryo heart fibroblasts in tissue culture medium containing 5γ/dl and also other chloroquine derivatives such as chloroquine diorotate, chloroquine chondroitine sulfate, chloroquine polygalacturonate showed same result in medium containing 2γ/dl of chloroquine too. 2) Furthermore, chloroquine diphosphate inhibited the growth of L strain cell in tissue culture medium containing 2γ/dl of chloroquine. 3) Effects of chloroquine on respiratory functions were observed in L strain cells with using Warburg's apparate, showing a decrease of oxygen consumption of the cells in the concertration of 2γ/dl of chloroquine. 4) Also fibroblast-inhibiting action was observed by administrating several drugs in the cultured chick embryo heart fibroblasts. The growth of the cells was not influenced by ACTH administration but several drugs such as tetrahydroxyquinone, phytonadione or oxophenarsine hydrochloride, arsphenamine sodium as well as prednisolone showed considerably potent fibroblast-inhibiting action. As mentioned above marked inhibition of the growth of fibroblasts was definitely demonstrated by chloroquine administration and this fibroblast-inhibiting action of chloroquine is thought to be important mechanisum of action to the treatment of bronchial asthma with lung fibrosis.