The preferential growth of B16 melanoma metastases in specific organs was studied. Following the i.v. injection of B16 melanoma cells into syngeneic C57BL/6 mice, tumor growths developed in the in situ lungs and in grafts of pulmonary or ovarian tissue implanted either s.c. or i.m. In contrast, neoplastic lesions failed to develop in control grafts of similarly implanted renal tissue or at the site of a surgical trauma. Parabiosis experiments suggested that the growth of the B16 melanoma in ectopic lung or ovary tissue was due to the immediate arrest of circulating neoplastic cells and not to shedding of malignant cells from foci growing in the in situ lungs. Quantitative analysis of tumor cell arrest and distribution using cells labeled with [125I]-5-iodo-2'-deoxyuridine indicated that the growth of tumors in the implanted organs was not due to an enhanced initial arrest of B16 cells. No significant differences in immediate tumor cell arrest were detected between implanted fragments of lungs (tumor positive) and kidney (tumor negative) or between organ-bearing and contralateral control limbs. We conclude that the outcome of metastasis is dependent on both tumor cell properties and host factors. This conclusion supports the "seed and soil" hypothesis to explain the nonrandom pattern of cancer metastasis.