Cayman’s Alanine Transaminase (ALT) Colorimetric Activity Assay Kit provides a convenient method of detecting ALT activity in serum, plasma, tissue samples, and cell lysates. Measurement of the ALT activity is carried out by monitoring the rate of NADH oxidation in a coupled reaction system employing lactate dehydrogenase (LDH). The oxidation of NADH to NAD+ is accompanied by a decrease in absorbance at 340 nm. Under circumstances in which the ALT activity is rate limiting, the rate decrease is directly proportional to the ALT activity in the sample.