Few human cell systems have been described in which a number of different genes induce transformation. The present investigation reports on our studies using primary human embryo retinoblasts as a model system to monitor transformation and the subsequent behaviour of individual transformants in terms of establishment, the frequency of immortalization and tumourigenic potential. SV40, Adenovirus E1 and E1A, and combinations of Adenovirus E1A and activated H-ras or N-ras were examined as transforming agents. Considerable differences were observed in the ability of these genes to transform human cells, to induce immortal lines and to produce cell lines with a tumourigenic phenotype. Activated ras genes were non-transforming in this system and the degree of complementation with adenovirus E1As in transformation experiments was dependent on both the adenovirus serotype and the ras gene used. The development of tumourigenic cell lines required the expression of more than one oncogene and additional genetic events were required in some in some instances before immortal cell lines were obtained. These findings contribute to the concept that the development of cancer is a multi-step process.