The difference in lactate dehydrogenase (LDH) activity between serum and plasma in postburn patients was investigated. LDH activity in the serum and plasma of 74 postburn patients was estimated using a CX-7 automatic biochemistry analyzer. The statistical significance of the difference was determined by Student's t-test. The results indicated that the LDH activity in plasma is significantly higher than that in serum (P<0.001). The higher LDH activity in plasma may be caused by leakage of LDH from ruptured platelets in the process of LDH determination. It is important to distinguish between serum and plasma LDH when considering test results as an aid to diagnosis.