In vivo surveillance of tumorigenic cells transformed in vitro.

Any theory of surveillance against cancer requires that cells susceptible to host protective mechanisms exist as intermediates on the pathway from normal to cancer. The failure to demonstrate a significant frequency of such intermediates as a result of chemical carcinogenesis has cast serious doubt on the validity of the surveillance hypothesis. Here we report the conditions in which such intermediates can be identified as the major class of transformed cells resulting from in vitro chemical carcinogenesis of a cloned fibroblastic cell line.