Subcutaneous tumors initiated with mouse B16 melanoma clones G3.5 and G3.12 disseminated visible spontaneous brain metastases in 67 per cent and 32 per cent, respectively, of mice with extensive lung metastasis. Most brain metastases appeared as pigmented emboli within blood vessels of the leptomeninges overlying the cerebral cortex. Intravascular metastases consisted of tumor cell aggregates surrounded by fibrous material and generally contained viable cells that proliferated in culture. Some metastatic emboli apparently proliferated intravascularly to such an extent as to cause vessel disruption, permitting tumor invasion into the adjacent cerebral cortex. Cultured cells from G3.12 leptomenings metastases produced tumors that metastasized to a much greater extent than unselected G3.12 tumors, but brain metastasis still occurred only secondarily, after initial dissemination to the lungs. In contrast, G3.5 brain metastasis-derived populations formed tumors that ultimately metastasized to the brain to lesser extents than did unselected G3.5 tumors. One selected variant, G3.12/BM2, reproducibly formed visible and viable brain metastases in more than 80 per cent of tumor-bearing mice, and lethal or potentially lethal brain metastases in 10-15 per cent of mice. This variant may serve as a model for clinical brain metastasis.