This research program was undertaken to improve the scoring of the transformation by chemical carcinogens of C3H/10T1/2 mouse embryo fibroblasts. (1) A probabilistic view of transformed focus formation in these cells induced by methylcholanthrene (MCA) treatment has been formulated and validated. (2) 5-Azacytidine induces differentiation of C3H/10T1/2 cells into both muscle cells and adipocytes. Although phorbol-ester related tumor promoters inhibit muscle cell formation, this is not affected by inhibitors of tumor promotion; moreover, other classes of tumor promoters do not inhibit this differentiation. (3) The powerful tumor promoter, 12-0-tetradecanoyl phorbol-13-acetate (TPA), produces temporary and reversible rounding up of the cells and loosened adhesion to the substratum. (4) A quantitative study of the 'natural history' of clones derived from various morphological types of transformed foci was carried out. (5) The cell-surface morphology was studied in the scanning electron microscope (SEM). (6) Preliminary experiments revealed that monoclonal antibodies common to transformed clones, which are probably against oncofetal antigens, can be prepared, and should be useful for scoring transformation. (7) Mouse peritoneal macrophages activated by BCG treatment selectively kill chemically transformed, but not nontransformed C3H/10T1/2 cells.