Chromosomal analysis and DNA flow cytometry were performed on HeLa, malignant fibrous histiocytoma (MFH) and their clonal sublines. Two HeLa clones attained confluency in a 60-mm dish on the 45th day after cloning. In MFH two clonal sublines began to increase cell numbers at the first. One continued growth and became confluent on the 177th day but the other ceased to grow. The chromosome number in all parent and clonal sublines showed a wide range of distribution, mostly in the hypotriploid range. In spite of single cell origen, remarkable karyological variation were seen in all clonal sublines. DNA histogram of all tumor lines measured by flow cytometry displayed distinct G1-peak which was narrower than expected from the distribution of chromosome numbers. All G1-peak as well as modal chromosome numbers were in hyportriploid region. From karyological variation in clonal sublines, chromosomal alteration may occur during growth. Furthermore, it is suggestive that the cell with near-triploid might be advantageous for proliferation, so that the cell with extremely small and large number of chromosomes can not progress.