Cryostat sections of 246 consecutive bone marrow biopsies from 212 patients with lymphoproliferative disease were investigated using a panel of monoclonal antibodies (MOAb's) and an immunoperoxidase technique. Bone marrow involvement was demonstrated by immunohistological examination in 121/160 patients (76%) with non-Hodgkin lymphomas (NHL) and 16/23 patients (70%) with plasma cell malignancies; the definite immunological diagnosis could be performed in 77% and 88%, respectively. Reactivity with the MoAb Ki-67 correlated with clinical parameters: in all cases exhibiting more than 5% positive cells an unfavourable course was seen, independent of the histological subtype. Another MoAb of potential prognostic relevance is KiM 4 b, which reacts with follicular dendritic cells (FDC). Besides the presence of FDC in germinal center tumors (CB/CC and CC-NHL) we found FDC in a minority of cases with B-CLL (5/44) and IC lymphoma (4/18). In the latter group 3/4 patients showed a favourable clinical course (vs 2/14 without FDC). The MoAb Tü 1 could discriminate between the lymphoplasmocytoid (11/12 positive) and the lymphoplasmocytic (0/6 positive) subtype of IC lymphoma and has proven of diagnostic importance. Expression of IL-2 receptors, detected by MoAb anti-Tac (CD 25), was demonstrated on leukemic cells from patients with hairy cell leukemia (100%), B-CLL (82%), IC (61%), CC (50%) and CB/CC lymphoma (50%). A considerable number of reactive T-lymphocytes (5%-60% of tissue cells) were identified among the neoplastic B cells with a predominance of CD4+ cells in most cases with NHL, whereas the CD4+/CD8+ ratio was significantly lower in myelomas and non-infiltrated bone marrows. The potential meaning of these findings is discussed. The immunohistological bone marrow analysis represents an important additional method in the diagnostic procedures of lymphoproliferative diseases involving the bone.