We have previously described a murine mammary tumor cell line (SP1) that metastasizes when transplanted into the mammary gland, but not when injected into the subcutaneous site. We used cytogenetic markers to assess genetic heterogeneity, and to monitor the selection and evolution of karyotypically distinct cell types during primary tumor growth and in metastases. The SP1 tumor cells are hypotetraploid (mean chromosome number = 72), and have at least four karyotypically distinct cell types. We found no consistent pattern of selection of tumor cell types in primary tumors. However, metastases were derived from a cell type that was present in the corresponding primary tumor. In addition, novel, karyotypically distinct cell types also appeared in the metastatic nodules. Markers that appeared in metastases included two translocations, t(10;18) and t(1;19). By injecting a mixture of cells from a metastatic nodule with a non-metastatic clone into mice, we showed that the new cell types in metastases displayed a stable increased growth and metastatic potential when compared to the non-metastatic clone, or when compared to the initial cell type from which the metastases derived. These results indicate that metastases are derived from a distinct cell type in the primary tumor, but that additional chromosome and cell evolution occurs, resulting in new cell types that are selected in metastases.