Two cell lines (SCL-I and SCL-II) derived from squamous cell carcinomas of human skin were investigated during 4 years in culture. Both lines were tumorigenic in nude mice, and cells could be recultivated from xenografts. Growth in agar remained poor, but both cell lines developed abnormal stratified epithelial structures in organotypical cultures. The morphological and particularly ultrastructural characteristics remained typical in both cultures and xenografts. Keratinization slightly decreased, but nude mouse tumors differentiated as the original tumors, and this was reflected in keratin expression. Six major polypeptides (Mr 61,000, 57,000, 54,000, 51,000, 49,000, and 45,000) were similarly identified in both tumors and cell lines, also after animal passage, which was further substantiated by two-dimensional gel electrophoresis, but quantitative variations were found with different growth conditions. A distinct keratin cytoskeletal network was visualized in both lines by immunofluorescence, but only a few cells in SCL-II also expressed vimentin. Flow cytometry demonstrated 2c DNA stem lines for original tumors and derived lines. Early passages were hypodiploid by cytogenetic analysis of banded chromosomes. In SCL-I, a shift to tetraploidy occurred before passage 20 and remained stable throughout. In SCL-II, an incomplete shift to near tetraploidy and a stem line deviation were apparent, but later passages, nude mouse tumors, and cells recultured therefrom were hypodiploid (2c) again. Chromosome studies further revealed distinct stable marker chromosomes which showed additional structural aberrations with time in culture and after animal passage. Thus, phenotypically and genotypically, each squamous cell carcinoma and its derived cell line were distinct, and characteristics were preserved over long time periods in vitro and through in vivo passage.