Metastatic dissemination of solid tumors in man and animals requires the successful completion of sequential steps. In addition to growth of the primary lesion, cells must intravasate, survive the transport process which may require the transversal of lymph nodes, arrest in a target organ such as the lung or liver, extravasate/establish in the parenchyma of the target organ, and replicate to form clinically significant metastatic nodules. the model we propose to develop will consist of subpopulations of a mouse mammary tumor which have selectable drug resistance markers that allow us to monitor the kinetics and route of metastatic dissemination. The recovery of colony forming tumor cells after plating enzymatically dispersed tissues in selective media provides a highly sensitive method to detect occult metastases in a quantitative manner. By transfecting a visual marker into the cells (E.coli B-galactosidase gene), dormant nonclonogenic cells may also be detected. A panel of metastatic subpopulations which metastasize via different routes (i.e. hematogenous and lymphatic routes) to different sites (i.e. liver and/or lung) and nonmetastatic subpopulations which fail at different steps in the metastatic sequence are to be derived and characterized. Several variants have already been obtained and partially characterized, illustrating the potential of this approach. These variant liens are easily maintained in cell culture, are readily sorted and recovered from liquid nitrogen, are highly tumorigenic, and are stable with respect to tumorigenic, metastatic, and drug marker properties. This model will ultimately allow one to pinpoint the effect of specific host immune mechanisms on specific stages of tumor progression and dissemination during metastasis.