Clones of the rat transplantable tumors: Walker's carcinosarcoma, rhabdomyosarcoma RA-2 and ovary carcinoma, were obtained by means of the lung colony technique and then transplanted subcutaneously and intraperitoneally. Growth parameters of the clones and their transplantants were measured (T, phi = ln2/T, V1). All the tumors gave populations of clones and transplantants with high variability of growth kinetics. There was a positive correlation between growth kinetics of the I and II generation transplantants, and the selection by growth kinetic parameters proved effective. The growth kinetic parameters seem to be quantitative indices with relatively low heritability, and populations of tumor clonogenic cells are heritably heterogeneous by their growth kinetic parameters.