Cellular H- and M-type lactate dehydrogenase (LDH) isoenzymes were determined immunohistochemically in a wide range of normal, inflammatory, benign neoplastic, and malignant tissues to assess the possible value of the isoenzymes in tumour diagnosis. Both types of LDH were detected almost ubiquitously in normal tissues. Increased staining intensity of LDH, especially M-type LDH, was demonstrated in most types of malignant tumours studied. Similar increased staining intensity of the isoenzymes was also noted to parallel the proliferative activity of the cells in non-neoplastic diseases and benign neoplasms. The results suggest that changes of cellular H- and M-type LDH isoenzymes are not specific for any type of malignant tumours. The increase in cellular H- and M-type LDH isoenzymes appears to be related to proliferation of cells in general.