The study was aimed at assessing the total enzyme activity and the profile of breast cancer and dysplasia on the human material. In addition, the validity of data was evaluated from the aspect of improving diagnostics. Lactate dehydrogenase activity, as well as the profile of its isoenzymes, pyruvate kinase and hexokinase, were measured. The study included 60 samples of breast cancer, out of which 20 were benign breast tumours and 40 were 1st and 2nd degree dysplasia of the breast. The samples were collected from the patients operated at the Institute for Oncology of Faculty of Medicine in Sremska Kamenica. Lactate dehydrogenase isoenzymes were separated by the vertical polyacrylamide gel disc electrophoresis according to the slightly modified Brewer and Ashworth's method. The activity of all the tested enzymes was measured under the conditions of linear kinetics in the function of time and enzyme concentration. Lactate dehydrogenase-5 was found in 88% of the analyzed breast cancer samples, whereas it was not detected in breast dysplasia. Pyruvate kinase (4.-isoenzyme) was about 50 times higher and the activity of hexokinase was 3 times higher in breast cancer than in breast dysplasia. Lactate dehydrogenase-5 and pyruvate kinase (4.-isoenzyme) are particularly important and reliable markers of malignity. The results obtained for quantitative and qualitative changes in the enzyme activity can be used to improve diagnostics and early diagnostics of malignant breast neoplasm.