Cell lines derived from C3H mouse ventral prostate grow to a monolayer, do not pile up, and do not give tumors on inoculation into isologous mice. When treated with carcinogenic hydrocarbons, these cells produce piledup colonies that give rise to fibrosarcomas in mice; these colonies are termed transformed. Individual single cells were treated with 0.5 per cent dimethyl sulfoxide (DMSO), and 2 per cent of them gave rise to clones containing transformed colonies. Under optimal conditions, treatment of individual single cells with 3-methylcholanthrene (MCA) led to the formation of transformed colonies in 100 per cent of the clones. From recloning experiments, it appears probable that all the progeny of the MCA-treated cells were potentially transformed, although the time required for the transformation to be scored was variable. It is concluded that in this system the carcinogen does not select for pre-existing malignant cells.