Fenbendazole is an anthelmintic drug widely used to treat and prevent pinworm infection in laboratory rodents. Data regarding possible side effects of fenbendazole on the immune system are conflicting, potentially due to the design of treatment protocols. The purpose of the current study was to determine the effects of 2 fenbendazole therapeutic regimens (continuous for 5 wk and alternating weeks [that is, 1 wk on, 1 wk off] for 9 wk) on the development of autoimmune disease in (NZB × NZW)F1 mice. No significant differences in survival curves or weight were observed between the treatment groups and cohort mice receiving nonmedicated feed. At the termination of the experiment, there were no differences in tissue pathology. Hematocrit decreased and BUN increased over time in all groups, but no significant differences were present between groups. After the cessation of treatment, mice fed the medicated diet continuously for 5 wk showed an increase in antiDNA antibody. Although this difference was significant, it did not affect survival curves or disease-related tissue or blood changes. These data indicate that common protocols of fenbendazole treatment do not alter the progression of autoimmune disease in (NZB × NZW)F1 mice.