The application of benzo[a]pyrene (BP) to normal hamsterembryo cells in culture produced transformed cells. These cells showed: 1) a hereditary random pattern of growth; 2) the ability to grow continuously in culture; 3) progressive growth as tumors after subcutaneous inoculation into adult hamsters; 4) a resistance to the toxic action of BP when tested at a later stage of growth in culture. Transformed cells in culture were obtained with the in vivo carcinogenic hydrocarbons BP, 3-methylcholanthrene (MCA), 7,12-dimethylbenz[a]anthracene, and 10-methylbenz [a]anthracene, but not in untreated hamster-embryo cultures or with the in vivo noncarcinogenic hydrocarbons 8-methylbenz[a]anthracene, chrysene, and pyrene. It was shown that carcinogenic hydrocarbons can directly induce in vitro the transformation of normal cells to tumor cells. Fibroblastic and epithelial-type cells were found in transformed cells in culture and in tumors produced by cell inoculation into animals. There was a high frequency of cell transformation, and with BP the maximum value obtained was 25.6 percent transformed clones. Application of BP at various times before clones were stained and examined indicated that between 1 and 2 days after addition of carcinogen are required for expression of the transformed state. This result suggests that the expression of transformation requires a process associated with cell division. Addition of BP to clones indicated that not all clones are equally transformable. Treatment of normal SWR mouse-embryo cultures with BP and MCA produced transformed cells, which were not found in short-term cultures of untreated controls. After longer periods in culture, clones with a random pattern of growth were also found in untreated controls, and both the treated and untreated mouse cells grew progressively as tumors after inoculation into adult SWR mice. For experiments without any appreciable spontaneous background of transformation, hamster cells are therefore preferable to mouse cells.