A distinctive B-cell has been recognized recently in reactive lymph nodes, especially those of toxoplasmic lymphadenitis. Previously designated as “immature sinus histiocytes” or “monocytoid” cells, these B-lymphocytes proliferate in subcapsular and parenchymal sinuses and the parafollicular area of nodes. The authors now report a 55-year-old male who developed a malignant lymphoma composed of cells with light microscopic, immunologic, and ultrastructural characteristics identical with these newly described B-cells. The term parafollicular B-lymphocytes (PBLs) is recommended herein to emphasize their morphologic and immunologic features. An unusual feature of this PBL lymphoma is the numerous benign-appearing hyperplastic follicles surrounded by the neoplastic infiltrate, mimicking the cytologic appearance and distribution of PBLs seen in toxoplasmic lymphadenitis. The function of these recently recognized B-cells is unknown; their anatomic relationship with hyperplastic follicular centers in reactive states and the lymphoma herein described suggests a role in follicular function.