Four spontaneously transformed keratinocyte lines (HELP I-IV) were raised from primary cultures of mouse epidermal cells grown on gas-permeable (Petriperm) dishes. Although tumorigenic, these cell lines still expressed the differentiated phenotype under mesenchymal influence in vivo in a fashion similar to normal cells and in contrast to previous observations on other transformed cell lines. Initially, after transplantation onto adult mice, HELP cells generally formed well organized ortho-keratinizing epithelia closely resembling those of normal epidermal cells. Later, dysplastic epithelia and papilloma-like structures developed and cells invaded subcutaneous host tissue. When injected subcutaneously into newborn syngeneic mice, all four cell lines gave rise to differentiated carcinomas at high frequency. Keratinized metastases were detected in the lung with HELP I, albeit at low frequency. Although the four HELP cell lines differed morphologically and biochemically in their degree of ortho-keratinization, no inverse relationship to their malignant potential was evident. In contrast to cell cultures, HELP transplants and tumors expressed epidermis-type "suprabasal" keratins. Metabolic labeling and electrophoresis on one and two-dimensional gels revealed both the basic 67 kilodaltons (kDa) and acidic 58 kDa components, including presumptive derivatives analogous to those observed in epidermal stratum corneum. However, associated with alterations in tissue architecture, the spatial control of keratin expression was gradually lost in papilloma-like and invading transplants and tumors, as demonstrated by indirect immunofluorescence microscopy (IIF). Thus, while cell differentiation appeared virtually normal, the progressive disturbances in tissue differentiation indicate important changes in the responsiveness of these malignant keratinocytes to environmental conditions.