Recently, we demonstrated that normal human epidermis contains IL1. The present work analyses further the significance of this observation by comparing the quantities of epidermal IL1 to those of 7 internal organs in normal rat. Epidermal PGE2 stimulating activity was 200 to 900 fold higher than in the internal organs. Similar results were obtained for the collagenase stimulatory activity and the comitogenic effect on thymocytes. Epidermal IL1 activity in membrane preparations was more than 100 times lower than the activity of the cytosol containing supernatant from the epidermal homogenate. Biochemical analysis by HPLC identified a MW approximately 17 Kd form with a pI 5.7 and a MW approximately 30 Kd form. These results 1) confirm the presence of high amounts of preformed IL1 in normal unstimulated epidermis, 2) show that, compared to 7 internal organs, the epidermis contains the highest activity, 3) suggest that epidermal IL1 is mainly situated in the cytosol, 4) identify a MW approximately 17 Kd form with a pI 5.7, as well as a MW approximately 30 Kd form which might represent an IL1 precursor, 5) demonstrate the copurification of PGE2 and collagenase stimulatory and thymocyte comitogenic activity and 6) give a starting reference for the in vivo study of IL1 activity in pathologic situations.