The purpose of this study was to develop an experimental model of squamous cell carcinoma that can be used to identify molecular and immunologic changes associated with primary events in malignant transformation, and those associated with metastatic tumor progression in the presence of host homeostatic and immunologic factors. Metastatic variants were derived following in vivo tumor progression of the in vitro transformed squamous cell carcinoma line Pam 212. The parental and metastatic cell lines exhibited similar morphologic features and molecular markers of an epithelial lineage, including an epithelial morphology in culture, cell surface expression of integrin alpha6beta4, and expression of mRNA of cytokeratins K6 and K14. When the growth and metastatic phenotype of the parental and reisolate cell lines was compared, the reisolate cell lines were found to exhibit a greater rate of growth and incidence of metastasis than the parental cell line when reimplanted in vivo. The difference in the growth rate of the parental cell line and the variants observed in vivo was not detected when growth of these lines was compared in vitro, suggesting that the growth advantage and selection of these variants requires tumor-host interaction. The metastatic variants exhibited a similar growth advantage in normal immunocompetent and SCID Balb/c mice, indicating that the growth advantage in vivo is not due to T or B lymphocyte-dependent immune factor(s). We conclude that metastatic variants derived following in vivo tumor progression of an in vitro transformed squamous cell carcinoma line exhibit a differential growth advantage in vivo that requires the host environment. Comparison of these in vitro transformed and in vivo derived metastatic variant cell lines with phenotypic differences in growth and metastasis should prove useful for dissecting the role of tumor and host factor(s) in malignant transformation and metastatic tumor progression of squamous cell carcinoma.