Based on the fact that lactate dehydrogenase (LDH) enzyme is a very sensitive indicator of the cellular metabolic state, aerobic or anaerobic direction of glycolysis, activation status, and malignant transformation, in this study we compared values of the spontaneous LDH release from circulating PBMC with sera LDH activity in 53 different subtypes of non-Hodgkin's lymphoma (NHL) patients. Results shows that serum LDH was significantly (p < 0.05) elevated in comparison to the range values only in the advance clinical stage (III and IV) in all investigated subtypes of NHL according to The Working and REAL classification. On the other hand, the spontaneous LDH release from cultures PBMC is significantly (p < 0.01) elevated in early and advanced stage in all investigated forms of NHL in comparison to healthy controls. Based on consideration that an increase in spontaneous LDH release appears before elevated sera LDH activity, we conclude that determination of spontaneous LDH release by microassay from cultured cells together with other findings may help in the diagnosis of NHL patients, especially in patients with early stage of disease.