Permethrin was applied to the shaved dorsal interscapular region of female C57Bl/6 mice at doses of 0.5 or 1.5 θ l/day in corn oil and neat 5.0 θ l/day. These doses corresponded to approximately 22, 66, and 220 mg/kg/day topical permethrin. Mice were exposed in this manner either daily for 10 or 30 consecutive days, or every other day for 7 or 14 exposures. Body weight was not affected by any of the treatment regimens. However, thymic weight was decreased and splenic weight was increased by 1.5 or 5.0 θ l permethrin/day, 2 days after termination of 10 consecutive days of topical chemical exposure. Cell surface antigen expression did not change in any treatment group on thymocytes (CD4, CD8), splenocytes (CD45R, Thy 1.2), or bone marrow cells (CD45, CD45R). A persistent, dose-related inhibition of the contact hypersensitivity (CH) response occurred in mice at all exposure levels of permethrin tested.