Patterns of Lewis lung carcinoma (LLC) cell invasion, colonization and metastasis were studied in C57 mouse uteri. LLC cells (5 X 10(4) in 0.05 ml) were infused nonsurgically into nonpregnant and pregnant uteri, 3 days postcoitum. The fate of cells was studied histologically on days 2, 5 and 18 posttreatment (PT). Despite a large degeneration of LLC cells in the lumina of both the pregnant and nonpregnant mice, in 38% of these uteri, tumor cells had invaded the endometrium by day 2 PT. However, subsequent distribution of tumor cells differed remarkably between the pregnant and the nonpregnant uteri. By day 5 PT, in 66% of the nonpregnant mice, tumor cells were common in the endometrium and metastases in other organs were seen in 40% of the animals. But in the pregnant mice only 40% uteri showed tumor cells and no metastasis was recorded. On day 18 PT tumor cells were rare in the nonpregnant uteri, but significantly, 75% of those animals showed lung and liver metastases. In pregnant mice, tumor cells neither survived in the uteri nor metastasized to other organs. LLC cells, infused into nonpregnant uteri, promptly metastasized to lungs without colonizing the uteri: this unique system may provide insight into the effects of organ-specific host factors on the growth and metastatic potential of tumor cells.