Congenitally immune-deficient bg/nu/xid (BNX) mice are severely compromised in their ability to mount T-cell, B-cell, and lymphokine-activated killer (LAK) cell responses. Successful engraftment of BNX mice with human hematopoietic stem cells has been demonstrated recently. We have investigated the potential use of BNX mice for studies relating to the biology and immunotherapy of human malignant melanoma. The intravenous injection of fresh single-cell suspensions of human malignant melanomas into mice resulted in widely disseminated disease. Metastatic spread of human melanoma in BNX mice mimicked that observed in patients: eg, there were numerous tumor nodules identified in the subcutaneous tissues as well as in a variety of visceral organs, including spleen, kidneys, thyroid, adrenals, lungs, heart, and brain. BNX mouse lymph nodes were replaced consistently by human malignant melanoma cells. The presence of human tumor cells in these mice was confirmed by histologic analysis and microcytofluorometry analyses using human melanoma-specific monoclonal antibodies (MAbs). Moreover, human melanoma cells passaged in BNX mice remained lysable in vitro by specifically cytolytic, autologous human tumor-infiltrating lymphocytes (TILs). The capacity of fresh human malignant melanoma to disseminate widely in BNX mice may prove valuable not only for study of the biology of metastatic spread but also for studies of the immunotherapy of human melanoma using melanoma-specific MAbs and chemotherapeutic agents, as well as human TILs and LAK cells with or without retrovirus-mediated gene transfer modification.