Oncogenic transformation of cultured cells is considered to be analogous to the induction of tumors in animals. Two properties of transformed cells: the capacity to grow in soft agar and morphological alterations were used as a basis for quantitative studies on the transforming properties of chemicals. The majority of experiments on chemical carcinogenesis were so far performed on murine, hamster and rat cell systems. The most pertinent questions to be resolved now are connected with malignant transformation of human cells.