The protease inhibitors alpha(1)-antitrypsin and antichymotrypsin are present in human milk, but little is known about their roles in protein digestion during infancy. It has been hypothesized that alpha(1)-antitrypsin and antichymotrypsin may modulate digestion in the infant gut.We determined whether the mammary gland expresses alpha(1)-antitrypsin and antichymotrypsin, measured alpha(1)-antitrypsin and antichymotrypsin throughout lactation, assessed the resistance of alpha(1)-antitrypsin to proteolysis, and determined the potential of alpha(1)-antitrypsin to affect the survival of other milk proteins.A pool of complementary DNA from the human mammary gland was analyzed with polymerase chain reaction to detect genes for alpha(1)-antitrypsin and antichymotrypsin. alpha(1)-Antitrypsin and antichymotrypsin concentrations were measured in milk samples obtained longitudinally (days 4-47) from 8 women. An in vitro model of infant digestion was used to assess the digestive stability of alpha(1)-antitrypsin against pepsin and pancreatin. Lactoferrin, with alpha(1)-antitrypsin present, was digested by pancreatin, and the digested proteins were separated.Alpha(1)-antitrypsin and antichymotrypsin concentrations were high in early milk and decreased throughout lactation. Polymerase chain reaction products were detected for both genes. After in vitro digestion, much of the alpha(1)-antitrypsin was still intact, whereas many other milk proteins were digested. Much of the lactoferrin was still intact after digestion, but only when alpha(1)-antitrypsin was added.The results suggest that alpha(1)-antitrypsin and antichymotrypsin are produced by the mammary gland and are present in milk in relatively high amounts in early lactation. alpha(1)-Antitrypsin may survive digestion and may affect the survival of other proteins.