A spontaneously immortalized human keratinocyte cell line is disclosed. In a preferred embodiment, this cell line is ATCC 12191. In another embodiment of the invention, a method of assaying the effect of a test tumor cell modulation agent is disclosed. The method comprises the steps of obtaining a human stratified squamous epithelial cell culture, wherein the culture comprises human malignant squamous epithelial cells and spontaneously immortalized human keratinocytes, wherein the culture forms a reconstituted epidermis. One then treats the epidermis with a test tumor cell modulation agent and evaluates the growth of the malignant cells within the epidermis.