The malignancy of different in vitro adapted cell lines was evaluated comparatively by the behavior of the cells when cultivated with fragments of chick mesonephros in organotypic cultures. These cell lines represented a wide spectrum of tumor-producing capacity when assayed in vivo by inoculation of isologous C3H or C57BL mice. The extent of infiltrative growth in the mesonephros fragments and the number of successful passages were considered as indicative of “malignancy” in vitro. There was good correlation between the data obtained from tests in vitro and in vivo for the six cell lines used, showing a wide range in degree of malignancy, from the low-cancer-producing N2 cell line, which grew in the mesonephros cultures only superficially and for few passages only, to the highly malignant N1, M6, or PTT-12 lines, which infiltrated extensively and appeared capable of growing indefinitely under these conditions.