The DNA content of murine fibrosarcoma cell lines of various metastatic potential was the subject of the current investigation. The cell lines were derived from methylcholanthrene-induced tumors as described previously (J. Varani et al., J. Natl. Cancer Inst., 71: 1281-1287, 1983). Cells were maintained in vitro and used for DNA studies no more than 48 h after passage. DNA staining was accomplished using propidium iodide and flow cytometry was used to quantitate relative amounts of DNA. Trout and chicken erythrocytes and mouse thymocytes were used as internal DNA standards for each cell line. DNA indices were calculated as the ratio of the G0-G1 peak channel number of the tumor cells to the G0-G1 peak channel number of the thymocytes. Manual chromosome counts were also obtained from each cell line using Giemsa-stained preparations. All cell lines demonstrated a single aneuploid population. The two tumor lines with the highest metastatic potential were slightly hyperdiploid whereas three low metastatic lines were near tetraploid. A sixth line of moderate metastatic potential was also found to be near tetraploid. Chromosome counts and flow cytometric analyses were in close agreement indicating that DNA content was largely due to chromosome replication. These data suggest that, in this model, metastatic potential and DNA content are inversely related once diploidy is exceeded.