BACKGROUND It is well known that human colostrum has important antiinflammatory functions. The purpose of the current study was to determine antiprotease levels in colostrum and serum and to assess the importance of local synthesis and the electrophoretic differences in both locations. METHODS Five protease inhibitors were determined by radical immunodiffusion in colostrum and serum samples taken simultaneously from 50 healthy women, 36 to 72 hours after delivery. RESULTS Antithrombin II, inter-alpha trypsin inhibitor, and alpha-2 macroglobulin levels were undetectable in colostrum. Mean antitrypsin levels in colostrum were 6% of serum levels, but colostrum alpha-1 antichymotrypsin was higher than expected (0.39+/-0.34 g/l) in relation according to the albumin passive transport, and their mean value was 41% of serum levels. Colostrum antichymotrypsin levels did not correlate with serum antichymotrypsin levels or with colostrum albumin levels. The antichymotrypsin molecule in colostrum had a slower electrophoretic mobility when compared with that of serum antichymotrypsin, and it showed a different pattern in Western blot analysis, with a predominating 80-kDa molecule. CONCLUSIONS Although the origin of colostral antichymotrypsin is unclear, local production in breast epithelial cells is likely. Antichymotrypsin is increased in colostrum, and its molecule has some characteristic differences, suggesting that it has an important and specific role in infant nutrition during breast milk feeding.