Physiological induction and reversal of focus formation and tumorigenicity in NIH 3T3 cells [published erratum appears in Proc Natl Acad Sci U S A 1991 Apr 15;88(8):3510] NIH 3T3 cells undergo morphological transformation in response to conditions of constrained growth, such as occur in low serum concentrations or at confluence. Transformation is expressed in a small fraction of the cells by the appearance of discrete foci of multiplying cells on a confluent monolayer of quiescent cells. We isolated and expanded cell populations from three dense and three light foci. Cells from each of these populations efficiently reproduced foci of the same morphotype when grown on a background of nontransformed NIH 3T3 cells. Using cultures derived from one of the dense foci (subline D/2), we found that the number of focus-forming units was stable and the cells remained tumorigenic when they were subjected to repeated thrice-weekly passage in 2% calf serum. However, equivalent passage in 10% calf serum eventually rendered the cells incapable of both focus production and tumor formation. The results show that the capacity to produce tumors as well as morphological transformation are produced as a response to physiological constraints of growth and/or metabolism in the absence of carcinogens and that both properties can be reversed by lifting the constraints. This behavior is typical of an adaptational response and, taken together with other supporting evidence, shows that tumorigenesis does not require conventional genetic alteration. 