A brain lymphoma cell line, designated TK, was established from biopsy material of a patient with primary intracranial lymphoma. The TK cell line grew in suspension with clumps of cells and consisted of primitive lymphoid cells. TK cells and original lymphoma cells from which the cell line was derived bore surface immunoglobulin and lacked Epstein-Barr virus-determined nuclear antigen. Transplantation of the TK cells into immunosuppressed newborn hamsters gave rise to invasive growth of tumors with metastasis to the brain and eyes. These findings provide definitive evidence for a B-cell origin of the brain lymphoma in this case.