A simple method for the determination of gamma glutamyltransferase (GGT) in completely haemolysed blood samples is described. Haemolysed blood was incubated in the presence of gamma glutamyl-p-nitroanilide and glycylglycine at 37 degrees C for 15 min, and the enzyme reaction was terminated by adding trichloroacetic acid. The pH in the supernatant was adjusted to 7.5 by using a buffer, and the amount of p-nitroaniline formed was measured spectrophotometrically. The correlation between this method and the GGT activity in serum as determined by the standard kinetic method was good, r = 0.982. The blood GGT activity (y) was related to the serum activity (x) as follows: y = 0.415x + 3.8. The effect of storage of blood at room temperature on the GGT activity was studied. The enzyme activity did not change during the first 12 days of storage.