A novel cultured cell line NOS-1, derived from a human oral primary squamous cell carcinoma (SCC) of the lower gingiva, was established without xenografting the tumor into nude mice by means of "Geneticin" treatment, which allowed for elimination of contaminated fibroblasts and produced enriched tumor cells at an early stage of the culture. NOS-1 cells showed numerous desmosome structures and some intermediate filaments. To determine tumorigenicity and to establish an orthotopic tissue invasion model for oral carcinoma, the NOS-1 cells were injected into the back and the tongue of male athymic nude mice. The back tumors showed an expansive growth pattern without significant invasion of surrounding tissues, while the tongue-implanted tumors exhibited invasive growth. The establishment of a novel oral primary tumor cell line and a new orthotopic tissue invasion model is expected to be useful for the study of biological characterization and for the identification of the invasive mechanism of human oral cancers.