This review summarizes findings from toxicologic, carcinogenic, immunologic, and metabolic studies on fenbendazole (FBZ). Currently, FBZ is used to treat or prevent pinworm outbreaks in laboratory rodents. Because antiparasitic treatments usually are not part of experimental designs, interactions from the medication on the outcomes of ongoing experiments are a concern. At therapeutic levels, FBZ does not alter the total content of cytochromes P450 but does induce certain hepatic cytochrome P450 isoforms, namely 1A1, 1A2, and 2B1. Although expressed constitutively at low or undetectable levels, these isoforms particularly are known for bioactivating a number of procarcinogens. Lifetime studies in rats have shown that FBZ is not a carcinogen but that it may behave as a tumor promoter when given after certain initiators. Unlike in other animal species, FBZ treatment-associated myelosuppression has not been reported to occur in rodents. The few currently available immunologic studies in mice, including an autoimmune model, have not shown effects on selected immune responses. However, data from other animal species suggest that the ability of B and T lymphocytes to proliferate in the secondary immune response may be suppressed during treatment with FBZ.