Loss of wild-type p53, either through deletion or mutation, has been demonstrated in most squamous cell carcinomas of the head and neck (HNSCC). Whether these mutant molecules contribute to tumor progression purely through loss of wild-type functions or by growth-promoting mechanisms, however, remains unclear. To begin to address these issues, we isolated a series of p53 cDNAs from HNSCC cell lines that contain missense or nonsense point mutations, insertions, or deletions. The ability of each of these molecules to transform NIH/3T3 cells to a malignant phenotype was assessed by stable transfection and expression under the control of a strong heterologous promoter. NIH/3T3 cells transfected with pLTR6p53, which harbors an H179L missense mutation, formed large tumors rapidly (in less than 4 wk) when transplanted to athymic mice, as did cells expressing pLTR13p53, which had undergone a V173F missense mutation and an in-frame deletion of 48 bp between codons 208 and 223. Cells transfected with pLTR17p53, predicted from the nucleotide sequence to encode a severely truncated p53 corresponding to the N-terminal 56 amino acids, also formed tumors. Cells transfected with pLTR15p53, which was predicted to encode a less severely truncated molecule, formed much smaller tumors and at lower frequencies. NIH/3T3 cells transfected with pLTR12p53 (exon 7 splice donor mutant), pLTRwtp53 (wild-type p53), or vector alone failed to form tumors for up to 2 mo after transplantation. pLTR6p53-transfected cells exhibited a highly malignant phenotype with invasion of regional lymph nodes, mediastinal and lung metastases, invasion of the abdominal wall, and dissemination throughout the peritoneal cavity. Histological assessment of the tumors revealed intensely vascularized fibrosarcomas with numerous cellular atypia, including frequent and aberrant mitoses. Tumor explants were recultured, and northern blot analysis of cellular RNA confirmed that the expression of exogenous p53 was maintained in each case. These data indicate that different p53 mutants contribute to tumorigenesis by specific mechanisms. Furthermore, the results obtained by using the pLTR17p53 transfectants imply that some truncated molecules may overcome the effects of wild-type p53 to contribute to malignancy.