We have developed four murine epidermal cell lines which form squamous papillomas when grafted to athymic nude mice in a reconstituted skin. Two of the lines, SP-1 and BP-4, were derived from pools of papillomas produced on SENCAR and BALB/c mouse skin, respectively, by initiation with 7,12-dimethylbenz(a)anthracene and promotion with 12-O-tetradecanoylphorbol-13-acetate. Line 308 was derived from BALB/c mouse skin initiated in vivo with 7,12-dimethylbenz(a)anthracene, culture of the epidermal cells, and selection of cells resistant to Ca²⁺-induced terminal differentiation. Line LC14 was derived from untreated, cultured newborn BALB/c mouse primary epidermal cells which spontaneously developed resistance to Ca²⁺-induced terminal differentiation. Each line has an activated ras^Ha gene with a mutation within codon 61. Cells from all four lines, in contrast to normal primary epidermal cells, survive in medium with Ca²⁺ levels > 0.1 mm. Clonal growth studies in culture showed a unique growth pattern for each of the four lines in medium with 1.4 mm and 0.05 mm Ca²⁺, with or without 12-O-tetradecanoylphorbol-13-acetate. Early passage cells of these lines should provide a valuable resource for detecting genes or genetic alterations which complement an activated ras gene to cause malignant conversion and for studying the biology of tumor promotion.