[en] The effects of radiation and culture conditions on the distribution of chromosome number in mouse Sarcoma 180 Y.S. cells were studied in respect to heterogeneity of their chromosome number and their tumorigenicity; four subpopulations separated by ficoll gradient centrifugation from in vivo cultured tumor cells demonstrated different tumorigenic properties and the hypotetraploid cells were found to be the most tumorigenic. (Author)