The occurrence of DP IV-reactive T-lymphocytes in patients with aleukemic non-Hodgkin's lymphoma (NHL) was studied using mononuclear cells (MNC) obtained from peripheral blood, bone marrow aspirates and lymph nodes, as well as cryostat sections of lymph nodes. These patients were found to have a reduced percentage of DP IV-reactive T-cells in MNC from peripheral blood (32%) in comparison to a healthy control group (44%). Patients suffering from chronic lymphocytic leukemia (B-type CLL) also had significant reduced level of DP IV-reactive cells (5% DP IV-reactive cells in the MNC fraction corresponding to about 40% of the T-cell fraction) in peripheral blood, whereas in lymph nodes of these patients 72% of the T-cells expressed DP IV. Among the MNC from lymph nodes of patients with aleukemic NHL about 24% were DP IV positive corresponding to 57% of the T-cell fraction. MNC from bone marrow of these patients were shown to contain about 21% T-lymphocytes and 18% DP IV-reactive cells. In addition to that, cryostat sections of lymph nodes were analysed for the occurrence and distribution of DP IV. In normal lymph nodes and tonsils DP IV-positive cells have the same distribution pattern like T-cells in general. In lymph node sections from patients with NHL of B-cell origin, such as B-CLL, the DP IV-reactive cells are sparsely and irregularly distributed and displaced by the proliferating B-cells. It is concluded that DP IV is a useful marker not only for T-lymphocytes in proliferating lymphatic tissue but also an indicator of imbalances between regulatory T-cell subsets in peripheral blood.