Transformation and neoplastic development of cells after exposure to chemical carcinogens in vitro were investigated. 4-Nitroquinoline-1-oxide and its carcinogenic derivatives successfully induced transformation and neoplastic development of hamster embryonic cells in tissue culture. Transformation occurred about 20–80 days after the first treatment with carcinogens, the earliest occurring at 15 days. The transformed cells were characterized by: proliferation in vitro, the cells forming a piled-up, dense layer; stable and unlimited growth in tissue culture; the ability to produce a progressively growing, transplantable fibrosarcoma when inoculated into hamsters. The shortest time at which progressively growing tumor was seen was 49 days after the first treatment, but histologic evidence of neoplastic development was observed as early as 23 days after treatment. In some cultures, neoplastic development was much later than the transformation of cells. The cultures of untreated cells contrasted strikingly with those of the treated cells, i.e., the control cultures had a limited lifespan and failed to produce tumor, except for one culture in which spontaneous transformation and neoplastic development occurred around 300 days in vitro.