In this method, blood is collected in ammonium heparinized microhematocrit tubes and lactate is directly determined in the plasma, separated within 15 min from the erythrocytes. Lactate is assayed by mixing 10 mul of sample with NAD+ and lactate dehydrogenase in tris(hydroxymethyl)aminomethane hydrazine buffer. The rate of increase in absorbance of the NADH formed, measured at 340 nm, is proportional to lactate concentration. The assay is complete in 4 min and absorbance is linearly related to concentration from 0.625 to 15 mmol/liter. Analytical recoveries of lactate added to plasma averaged 104% (range, 91-116%). Results compared well for plasma samples analyzed by this method with the CentrifiChem and the Du Pont aca.