Pinworm infection (oxyuriasis) is a common problem in rodent colonies. Facility-wide prophylactic treatment of all mice with a diet containing therapeutic levels of fenbendazole for several weeks is often used to control pinworm outbreaks. We examined the effect of feeding a therapeutic diet containing 150 ppm fenbendazole on the growth of EMT6 mouse mammary tumors implanted into BALB/c Rw mice. Mice were randomized to receive either a fenbendazole-containing or control diet for 1 wk before tumor cells were injected intradermally in the flanks and throughout tumor growth. Tumor growth was monitored by serial measurements of tumor diameters from the time tumors became palpable until they reached 1000 mm3. The medicated diet did not alter tumor growth, invasion, or metastasis. When tumors reached volumes of approximately 100 mm3, some were irradiated locally with 10 Gy of X-rays. Irradiation significantly delayed tumor growth; fenbendazole did not alter the radiation-induced growth delay. However, cell culture studies showed that fenbendazole concentrations not far above those expected in the tissues of mice on this diet altered the growth of the tumor cells in culture. Recent data from other laboratories also have demonstrated effects of fenbendazole that could complicate experiments. Care should therefore be exercised in deciding whether chow containing fenbendazole should be administered to mouse colonies being used in cancer research.