Fenbendazole (FBZ) is an anthelmintic drug widely used to treat and prevent pinworm outbreaks in laboratory rodents. Although data in nonrodent species indicate possible effects of fenbendazole on the bone marrow and lymphocyte proliferation and function, little has been reported regarding possible effects on the rodent immune system. The purpose of the current study was to determine the effects of a therapeutic regimen of FBZ on immune parameters in BALB/c mice. Both 9-wk on-off and 5-wk continuous medicated feed protocols were assessed. No significant differences between normal and FBZ diet treated mice were observed in the following parameters: complete blood count, blood chemistry, quantitation of major T and B cell markers in spleen, quantitation of T cell markers in the thymus, spleen cell proliferation to T and B cell mitogens, bone marrow colony-forming cell assays, skin graft rejection, and primary and secondary humoral immune responses. These data indicate that FBZ treatment does not affect many standard broad measures of immune function.