Spontaneous metastasis of subcutaneous B16 melanoma transplants proceeds in two distinct stages: initially to the lungs, and secondarily, following tumor removal, from established lung metastases to extrapulmonary systemic sites. Coincident with extrapulmonary metastasis, there is a dramatic amplification of visible lung metastases, with death generally resulting from extensive lung metastasis. The progression of lung metastasis, and lung colonization initiated by intravenous injection of tumor cells, was investigated using B16 melanoma clone G3.12. Analysis of the growth of invisible metastases in organ culture explants of lung revealed that tumors continually disseminated relatively small numbers of lung metastases after reaching a size of about 6 mm in diameter. However, most terminal-stage lung metastases, along with all extrapulmonary metastases, apparently arise from a secondary spread of tumor cells from tumor-derived lung metastases 1-2 mm in size. Individual lung colonies, initiated with G3.12 cells bound to single microbeads, also disseminated large numbers of secondary lung colonies, as well as extrapulmonary colonies, at a 1- to 2-mm size. The mechanism for intrapulmonary spread of secondary metastases and colonies is unclear, but the consequence appears to be a secondary stage of intrapulmonary and extrapulmonary metastasis with selection for tumor cells with rapid growth rates.