Abstract: Objective To study the isoenzyme differences between normal and elevated serum lactic dehydrogenase(LDH)in different tumor patients. Methods We determined the serum LDH isoenzyme activitise in 309 tumor patients with normal LDH and 276 with elevated LDH,and in 37 normal controls using agarose gel electrophoresis. Results Different tumor patients with normal serum LDH activities had elevated LDH3 activity and there was significant difference between this group and the normal control.Patients with elevated serum LDH activities had elevated LDH3,LDH4,and LDH5 activities and there was significant difference compared with the normal control. Conclusion LDH isoenzymes may be used as a reference diagnosis index for the malignant tumor patients,Especially,the tumor patients with normal;LDH acticities had abnormal LDH isoenzyme pattern.Elevated LDH3 was one of the early characteristics of tumors.