Abstract A comparison was made of the in vivo growth potential of tumor cells derived from mouse mammary adenocarcinomas arising in premalignant outgrowth lines D 1 and D 2. Twenty-one tumors were dissociated and cultured as cell monolayers in vitro. Three to four days later 50,000–100,000 cells were inoculated intravenously into syngeneic female BALBlc mice. Primary cultures of tumor cells which arose in D 2 HAN outgrowths varied considerably in their ability to form tumor nodules in the lungs of syngeneic mice 4 weeks post-inoculation. An increase in either the number of cells inoculated or in the length of in vivo incubation influenced the extent of tumor growth observed in the lungs. Primary cultures of tumor cells which arose in D 1 HAN outgrowths inoculated at the same cell number rarely formed tumor nodules in the lungs 4 weeks post-injection. If, however, twice as many cells were introduced, a small number of large nodules were observed within 4 weeks. An epithelial cell line (WAZ- 2T) isolated from D 1 tumor tissue produced small lung nodules by 4 weeks, but larger more numerous nodules arose when in vivo incubation was extended an additional 3 weeks. This study indicates that primary D 2 tumor cells have a variable but higher degree of metastatic potential than primary D 1 tumor cells following intravenous inoculation. Tumors which develop in the D 2 and D 1 premalignant lines may be useful for studies of chemotherapy and cell latency of tumor metastases and the identification of the specific cell subpopulations involved in these events.