Previous studies have shown that prenatal exposure to the organochlorine pesticide chlordane significantly decreases the ear swelling response to the contact allergen oxazolone in BALB/c mice. Alterations of macrophage function in the efferent arm of the contact hypersensitivity response have also been reported. In the current study, chlordane was applied topically and the effects of oxazolone-induced contact hypersensitivity were determined. Initially, the reduction in oxazolone-induced ear swelling in topically-exposed female BALB/c mice was compared to 30-day-old BALB/c female mice exposed prenatally to chlordane. Prenatal chlordane exposure induced a 36% reduction in ear swelling compared to a 60% reduction following topical treatment at the challenge phase. Topically-applied chlordane also reduced the oxazolone-induced ear swelling by 40% when applied at sensitization. When applied at both sensitization and challenge, ear swelling was reduced by 71%. In a time-course study, it was determined that chlordane must be applied at the time of sensitization, challenge or both or within 1 h post-challenge to significantly reduce ear swelling. A dose-response study showed that the lowest concentration of chlordane resulting in a significantly reduced ear swelling response was 20 micrograms per ear.