The enzyme lactate dehydrogenase (LDH) activity of peripheral blood mononuclear cells (PBMC), LDH isotype H and M pattern and PBMC spontaneous LDH release activity were examined in 55 non-Hodgkin's lymphoma (NHL) patients, 46 Hodgkin's disease (HD) patients and 47 controls. The intracellular LDH and M isotype activity of PBMC, their spontaneous LDH release activity significantly increase (p < 0.01) in NHL with progressing histological grade of malignancy. Contrary to this, all classical HD patients have a significant elevation (p < 0.05) of each of these parameters. Furthermore, unlike HD, in NHL clinical stage is associated with significant (p < 0.05) increase in the level of spontaneous LDH release activity in each histological form. It is also shown that spontaneous LDH release activity of PBMC for HD and NHL patients demonstrates significant positive correlation (p < 0.005) with serum LDH level, although elevation of spontaneous LDH release precedes serum LDH increase in both diseases. The results obtained regarding alterations in intracellular, isotype and spontaneously released LDH activity of circulating PBMC show that these parameters are dependent, in NHL patients, on the grade of malignancy and tumor burden, while they are persistently present in HD patients.