With the use of the rat 13762 mammary adenocarcinoma tumor, an animal model for spontaneous mammary tumor metastasis was developed. The parental tumor implanted sc into the mammary fat pads of female Fischer 344 rats metastasized to other organs. Oophorectomy did not modify the metastatic properties of the parental 13762 tumor. Cell lines were adapted to tissue culture from lymph node or lung metastases, and these were compared to parental tumor transplants in spontaneous metastasis assays. Only the cell lines established from metastases were spontaneously metastatic within 23 days after sc implantation, indicating that tumor cell populations from metastases are more metastatic than are cells from the parental tumor. Several individual parental tumor-derived and lung metastasis-derived clones were compared for spontaneous metastatic potentials, cell culture morphologies, histologic structures at primary implant and secondary metastatic sites, and growth characteristics in vivo and in vitro. There was no correlation between any of these tumor and cellular properties and metastatic potential. The parental mammary tumor-derived, lymph node metastasis-derived, and lung metastasis-derived cell lines and clones with differing spontaneous metastatic properties should prove useful in studies on the roles of tumor cell and host properties in lymphatic and blood-borne metastasis.