Increased gamma-glutamyl transpeptidase (GGT) levels have been observed in patients with several types of liver diseases. However, since GGT activity can be inhibited or increased by several substances, the interpretation of these increased levels should be made with caution. The present study was designed to determine plasma GGT activity in blood samples obtained from 24 adult volunteers (13 males and 11 females aged 20 to 40 years) with the use of three different anticoagulants, i.e., ethylenediaminetetraacetic acid (EDTA), calcium fluoride and calcium citrate, and to compare it with GGT activity in serum obtained from the same blood samples. Serum GGT activity was significantly higher than plasma GGT activity (P less than 0.001) and significantly higher in men than in women (P less than 0.05), though plasma GGT activity was similar for both sexes.