Thirty-six lung adenomata induced in mice by urethane followed or not by cortisone, all had an adenomatous morphology at first s.c. transplant in syngeneic hosts. Seventeen of them acquired a sarcomatous structure within a few s.c. transplant generations whilst the other 19 remained adenomatous for as long as tested, i.e. at least 10 transplant generations. The change of morphology was not dependent on s.c. growth, since tumours also transformed when allowed to grow in the lung, and was not correlated to the capacity of a tumour for growth or metastasis. The 2 types of tumours were antigenically different, since only tumours that after few transplants changed their morphology were found at the first s.c. transplant to possess tumour-associated membrane antigens as revealed by an in vitro test. In addition, only the tumours which acquired a sarcomatous morphology were found gs-positive. The majority of antigenic primary tumours arose in mice belonging to the groups of treatment which induced the strongest immunodepression. It is suggested that a predisposition to sarcoma progression is related to an immunological control, at the time of adenoma induction, of an oncornavirus, responsible for the superimposed sarcomatous change.