The major concern of this study was to clarify how to evaluate serum lactate dehydrogenase (LDH) as one of an index to assess a therapeutic effect for malignant neoplastic disease, and also to seach for a possible difference in the quantitative and/or qualitative alteration of this enzyme according to the mechanism by which tumors loose their biological potency in the host. In the experimental study, total LDH activity and its isozyme fraction value were determined following the complete surgical removal and ischemic necrosis of methylcholanthrene-induced mouse sarcoma. In the clinical study, alterations of activity of total LDH and isozyme fraction III (LDH III) and IV (LDH IV) were observed before and after the treatment, as possible induces for therapeutic effects on 98 patients with various malignant neoplasm, based on the result obtained in the experimental study. The result was summarized as follows: 1) In the tumor bearing mice, LDH activity was significantly higher than in the normal mice as had been reported by many others, and an remarkable elevation of LDH III and LDH IV was also noted. 2) Following the surgical removal of the tumor, LDH activity showed no concomitant decrease and the elevated level was maintained for at least two weeks without diminishing down to that in the normal mice, whereas LDH III and IV dropped early within the first week to that of the normal control. 3) Following the causation of ischemic necrosis of the tumor, LDH activity showed further increase temporarily in the first week and gradually decreased to the level of the tumor bearing mice by the end of the second week, and also LDH III and LDH IV descended to the normal level antecedent to that of the total enzymatic activity, but with milder slope than in the tumor-removal group. The temporary increase in these value was not demonstrated even in the early stage of the post-necrotic course. 4) The average elevation rate of LDH activity in total 98 preoperative patients with malignant tumor was 20 % , and the activity was further raised even after the radical surgery for the tumor, although it returned approximately to the preoperative level by the end of the 4th week in more than half of the patient. Exceptional decrease to the normal level was observed in some of the patients with cancer of the breast and rectum, but none in those of the gastric and lung cancer. 5) The characteristic elevation of LDH III and/or LDH IV was observed among the cancer patients, according presumably to an organ-specificity from which the tumor developed. After the radical surgery, both of them lowered their activity while the total enzymatic activity still maintained the elevated level, and this occurred earlier, more precisely and with higher incidence in LDH N than in LDH III. This trend was more clearly demonstrated in Stage II than in other stages. 6) In those who received chemotherapy and/or radiotherapy, activity of total LDH, LDH III and LDH IV showed a definite reduction with quite a few exceptional case, when a therapeutic effect was clinically manifested.