Intratumoral injection of I-125 iododeoxyuridine (IUdR), saline solution, and oil suspension was investigated using Ehrlich ascites tumors in the thighs of mice. The oil suspension was more effective in tumor growth delay than was the saline solution. Single injection of the oil suspension at the dose of 12.5 microCi resulted in 21.5 days growth delay, whereas 50 microCi of the saline solution resulted in 11.5 days growth delay relative to control growth delay. At 40 days after treatment, higher radioactivities were observed in the tumor and the skin of the mice treated with the oil suspension, which represented the prolongation of I-125 IUdR oil suspension within the tumor. No normal tissue toxicities were observed.