The fate of clonal lineages in tumor formation and metastasis has been studied by genotypic marking of cells from three separate tumor lines of different malignant potential. Marking was accomplished by random incorporation of the neomycin resistance gene and visualized by Southern blot analysis of integration sites. Primary tumors formed by polyclonal cell suspensions of all three cell lines injected s.c. usually remained polyclonal even at late stages of tumor growth and metastatic spread. Lung metastases were often clonal, but it was not unusual to find ones of polyclonal origin. Lymph node metastases were almost always polyclonal and remained so, as they grew large. Sometimes clones present in the original inoculum were absent in the primary. Other times clones visible in the metastases were undetectable in the corresponding primary tumor. Occasionally a single clone became dominant in the primary, and others were eliminated, but this was not a necessary prelude to the onset of invasive or metastatic behavior. It is concluded that there is considerable variation in the results obtained with various cell lines in different circumstances. Even clones which are underrepresented in the original inoculum or the primary tumor can acquire metastatic capability. Hence, progression of malignancy is not uniformly dependent on prior or concurrent extinction of other non- or less metastatic clones in the neoplasm, and the underlying mechanisms of invasion and metastasis can be separated from those which sometimes confer growth supremacy on a clone of tumor cells. The frequent continuing genetic heterogeneity of cells in a neoplasm has substantial implications for clinical treatment protocols.