.gamma.-Glutamyl transpeptidase (GGT) activity in serum was increased in the majority of women with viral hepatitis occurring in the 1st half of pregnancy. During the second half of gestation GGT activity was abnormal less frequently and the mean value was relatively depressed, even though hepatitis was as severe. Mean GGT activity was also lower, and abnormal values were less frequent, in nonpregnant women with viral hepatitis who were taking birth control pills (BCP). Depressed GGT is not attributable to an inhibitor in serum in women in late pregnancy or taking BCP. Estrogen and/or progestational compounds possibly affect liver such that less GGT is released into blood with acute hepatocellular injury. Hyperbilirubinemia was also associated with depressed serum GGT activity; bilirubin added to serum in vitro interfered with measured activity of the enzyme.