Malignant lymphomas have traditionally been classified on solely morphologic grounds. With new immunologic and cytochemical techniques, it has been possible to characterize normal cells of the T-lymphocytic, B-lymphocytic, and monocyte-macrophage systems. Application of these methodologies to malignant lymphomas has established their nature as neoplasmas of the immune system. Within the B-lymphocytic system it is possible to identify subpopulations responsible for Burkitt's tumor, follicular (nodular) lymphomas, lymphocytic lymphomas of intermediate differentiation and well differentiated lymphocytic lymphomas. The T-lymphocytic system includes lymphoblastic lymphomas, mycosis fungoides, and Sezary's syndrome. Large cell lymphomas are diverse but the majority are tumors of transformed lymphocytes, usually of the B-lymphocytic system. The precise nature of the neoplastic cells of Hodgkin's disease, i.e., Reed-Sternberg cells and their mononuclear counterparts, has not yet been established. Despite previous suggestions of a B-lymphocytic or T-lymphocytic origin, recent studies utilizing in vivo cultivation have strongly suggested derivation from the monocyte-macrophage system.