This study was performed to find a reliable method to measure lactate dehydrogenase (LDH) activity in blood samples. Human plasma contains thrombocytes whose number depends on the speed of centrifugation applied to the freshly-drawn blood sample. They do not disturb the measurement of LDH in plasma, provided the plasma sample has not been subjected to procedures that may destroy the integrity of the thrombocytes, e.g. standing overnight at 4 degrees C, freezing, or treatment with ultrasonic waves. Centrifugation of plasma at a minimum of 1174 x g eliminates the presence of thrombocytes. The serum of healthy human subjects invariably contains higher LDH activity than the plasma: the difference is 30 +/- 18 U/l (mean +/- S.D.). LDH-isoenzyme analysis shows that a large part of the LDH activity difference is caused by lysis of erythrocytes in the clot. Liberation of 30 U LDH per litre is not associated with visible hemolysis.