Based on the possibility of tumor necrosis factor (TNF)-α to perform multiple and opposite biologic effects, we simultaneously investigated in vitro its effects on intracellular lactate dehydrogenase (LDH)-H and LDH-M isoenzyme activity and morphological characteristics following induction of apoptosis in peripheral blood mononuclear cells (PBMC) of non-Hodgkin's lymphoma patients (NHL) prior to and after the end of applied chemotherapy. TNF-α showed a significant increase (p<0.05) of LDH–H and LDH-M activity in sonified PBMC of healthy controls after 18 h cultures accompanied with an increase of apoptotic index (AI) from 2.3 to 16.2%. Contrary to this, in PBMC of NHL patients prior to therapy TNF-α induced a significant decrease (p<0.05) of LDH-H isotype activity. In patients after administration of chemotherapy, TNF-α in a dose of 100 U/ml induced a significant increase (p<0.05) of LDH-M isotype activity, but not of LDH-H. In the PBMC of NHL patients prior to chemotherapy, TNF-α in vitro induced an increase of AI from 2.8 up to 6.8%, while in PBMC of NHL patients after applied chemotherapy AI changed from 7.2 to 14.4%. However, there was no significant difference in the increase of apoptosis in PBMC of NHL patients with high-grade malignancy and high rate response among patients who received first-line therapy, high-dose therapy, or third-line therapy regimens after in vitro TNF-α treatment. These results indicated different susceptibilities of PBMC of NHL to TNF-α when effects were analyzed by determination of intracellular LDH isotype profile and induction of apoptosis prior to and after administration of therapy in comparison to effects on healthy controls PBMC.