Mast-cell-derived mediators showed mitogenic activities on mouse-transformed epidermal cell line Pam 212 cells. These activities were eluted into the low-molecular-weight fractions below a molecular weight of 10 kD on a high performance liquid chromatography TSK 2000G column, and were partially abrogated by antihistamines or anticytokine antibodies, including anti-IL1 alpha, -IL1 beta or IL6 antibodies. Pretreatment of mast cell lines with sodium butyrate enhanced the production of these factors. Calcium ionophore or Concanavalin A (ConA) stimulated mast cells to generate factor production. These results suggest that mast-cell-derived mediators might play some role in epidermal hyperplasia seen in lichenified lesions in atopic dermatitis.