In an in vitro study, intended to develop tumoricidal therapy with the use of human leukocyte, an interesting phenomenon was found. Normal human polymorphonuclear leukocytes (PMN) plated on a cell line established from malignant trichilemmal cyst (DJM-1), a kind of squamous cell carcinoma, in a serum-free media and incubated for 48 h induced the detachment of DJM-1. The detachment was more extensive as the number of PMN increased. The detachment rate was 97.0% when the number of PMN and DJM-1 in a well was in a ratio 2.4:1 and the viability of detached DJM-1 was 96.5%. Two kinds of proteinase inhibitors, especially the inhibitor of neutrophil elastase, fetal bovine serum, and monoclonal anti-laminin antibody inhibited the detachment significantly. Furthermore, when PMN were seeded in a chamber with a filter membrane bottom to prevent direct contact with DJM-1, DJM-1 detachment decreased to 14.2%. In view of these results, the following mechanism was postulated. Activated by their adhesion to DJM-1, especially between laminin receptor on PMN and laminin on DJM-1, PMN secreted proteinases, resulting in DJM-1 detachment. This phenomenon might be an expression of cytotoxicity of PMN to cancer cells, because cultured cancer cells of epithelium origin such as DJM-1 can grow only after they are firmly attached to the substratum. This phenomenon, in turn, may explain the final step in the induction of epidermal-dermal separation in subepidermal bullous diseases with PMN infiltration such as bullous pemphigoid and dermatitis herpetiformis if we could regard DJM-1 as normal keratinocyte.