It has been possible to reinterpret non-Hodgkin's lymphomas by applying subtle histological, cytological, and cytochemical techniques and electron microscopy as well as by using modern immunological methods and immune chemical analyses. This has led to the differentiation of four main groups of low-grade and three main groups of high-grade malignant lymphomas. Most non-Hodgkin's lymphomas are derived from the B-lymphocyte system. Only some of the lymphocytic, a small fraction of the lymphoblastic, and very few immunoblastic lymphomas originate from the T-lymphocyte system. True reticulosarcomas are very rare and will have to be redefined.