The tumor suppressor protein p53 shows growth and transformation suppression functions that are frequently lost by mutant proteins detected in cancers. Using a large series of p53 mutants, we have demonstrated an excellent correlation between transcriptional activation and growth suppression in p53-null human cells. Not all transcriptionally active mutants retain the ability to suppress transformation in primary rodent cells, however, and two tumor-derived point mutants displayed some evidence of both transforming and transactivating activity. Transformation by these mutants was not mediated by transdominant repression of endogenous p53 transactivating function, and cell lines expressing these p53 proteins showed elevated p53 transcriptional activity. Our results suggest that activation of transcriptional regulation by p53 will not necessarily result in tumor suppression.