Chitin is a polymer of D-glucosamine, beta-poly-N-acetyl D-glucosamine. The chitin derivative, 70% deacetylated chitin (DAC-70), is immunopotentiating. The present study was conducted to determine the immunoregulatory events in skin following topical application of DAC-70 to DNCB-sensitized contact dermatitis in guinea pigs. On the 3rd day of treatment, DNCB-induced erythema was reduced in all five guinea pigs treated with 10% or 3.5% DAC-70 ointment. On the 7th day of treatment, ATPase-positive Langerhans cell (LC) density in epidermal sheets was 1021 + 131/mm2(mean + SD) for 10% DAC-70-treated skin, and 1000 + 130/mm2for 3.5% DAC-70-trèated skin, both values exceeding 782 + 65/mm2for normal epidermis. The value in DAC-70-treated samples was significantly greater (P<0.01) than in normal epidermis. DAC-70 thus appears to have an immunoregulatory effect by increasing the number of epidermal LCs.