In 21 cancer patients receiving administration of FT-207 suppositories, we studied the concentration of FT-207 and 5-FU in the blood before surgery, and in the blood and tissues-tumor, normal and lymph node-sampled during surgery. These 21 cases were comprised of 12 cases of stomach cancer, 1 case of lymphosarcoma, 4 cases of rectal cancer, 2 cases of sigmoid cancer and 2 cases of breast cancer. Two hours after administration, the concentration of 5-FU reached its highest level and continued to maintain a level at which it could be expected to bring about clinical results even after four hours. The concentration of 5-FU in tumors and lymph nodes was high, especially in tumors compared with normal tissue (p less than 0.01). No difference could be found in the concentrations of FT-207 and 5-FU in serum and tissues from each 12 cases of stomach cancer, not only between of the early cancer and advanced cancer but also between patients more than 60 years old and those under 60 years old. No side-effects occurred in any case during suppository administration.