Abstract: It has been found that water-insoluble, thermogelable .beta.-1,3-glucan produced by microorganisms, lower polymers obtainable upon partial hydrolysis of the glucan or, carboxymethylated derivatives of said glucan or of said lower polymers are useful for inhibiting growth of tumors in a warm-blooded animal. The inhibitory activity against various tumors of these polysaccharides is very strong and significant whenever administered to warm-blooded animals from the early to very late stage of tumor progression or even when administered prior to plantation of tumors. The carboxymethylated derivatives above-mentioned are novel water-soluble compounds which can be produced by carboxymethylation of said glucan and partial hydrolyzate thereof.