A comparative study of two different species of a fluorouracil assay was conducted on 11 patients who had carcinoma that was deemed unresectable after the surgical operation. For these patients FU at a dose of 10 mg/kg was intravenously administered before operation, and portions of the tumors were resected within 120-150 min to assay both the (FU)RNA/RNA and FU/protein. After surgery, all patients were given FU alone either intravenously or orally. The FU, which was in an acid-soluble material (FU/protein), was not related to the antitumor effect of FU. However, the FU in RNA [(FU)RNA/RNA)] was found to be related to the antitumor effect of FU. When the concentration of (FU)RNA/RNA was above approximately 200 ng/mg, FU was effectual in unresectable carcinomas. It is probable that the (FU)RNA/RNA may be more suitable than FU/protein for predicting the antitumor effect of FU.