FT-207 was administered to 30 patients with squamous cell carcinoma of the head and neck. After FT-207 administration tumor concentration of FT-207 and 5-FU was measured using a chemical assay method. Differences of FT-207 and 5-FU in total doses, administration routes (oral, intrarectal, and intravenous), tumor sites (larynx, maxillary sinus, tongue, neck and hypopharynx), and types of differentiation (well-differentiated, moderately-differentiated, and poorly-differentiated) were studied and the following results were obtained: 1. In FT-207 tumor concentration study, no significant difference was observed in any group. 2. 5-FU tumor concentration increased in higher total administration doses and in advanced differentiated type groups as well as in the non-irradiated group. 3. 5-FU tumor concentration was measured by T/N ratio; oral and intravenous administration methods showed 4.5 (P less than 0.05) and 2.4 (P less than 0.10) respectively. In larynx tumor the highest concentration of 3.9 (P less than 0.05) was obtained among various tumors. The value of 4.0 (P less than 0.05) was yielded in the well-differentiated type, which was significantly higher compared to that of other differentiation types. Overall T/N ratio (5-FU concentration in the tumor/5-FU concentration in the normal tissue) was 2.7 (P less than 0.025).