We have established conditions for the immortalization of human fibroblasts by the large T antigen of the rodent virus polyoma. This allows the mechanism of immortalization to be studied, without interference by transformation events, in cells with relatively stable chromosomes. Large T antigen could immortalize human fibroblasts if expression was driven by a heterologous promoter like the immediate early promoter/enhancer of cytomegalovirus or the inducible mouse mammary tumour virus (MMTV) promoter. Using the latter promoter and dexamethasone, three clones were obtained, the immortalized phenotype of which was strictly dependent on the induction of T-antigen expression. At least one of these clones became mortal after removal of the inducing agent. The expression of large T antigen was paralleled by PCNA gene expression, as shown by nuclear run-off transcription, whereas none of a number of other known proto-oncogenes was influenced in its activity. Immortalized fibroblasts were readily transformed by polyoma virus middle T antigen expressed from the MMTV promoter or by the activated c-Ha-ras oncogene. The reversibility of immortalization and transformation is considered.