The trypsin-inhibitor capacity of bovine milk was seen to increase in mastitis. This capacity showed good correlation with the California Mastitis Test score, somatic cell count (Counter method) and bovine serum albumin (BSA) content (radial immunodiffusion). The elevated antitrypsin level proved to be a safer indicator of mastitis than BSA alone. Using gel-filtration chromatography, the major trypsin-inhibitor activity of mastitic milk and plasma was eluted with BSA, indicating a similar mol. wt (70 000). This fraction showed electrophoretic polymorphism. A large molecular weight fraction cross-reacted immunologically with human alpha 2-macroglobulin. The antitrypsin activity in mastitic milk was clearly different from the principal activity in colostrum, which had a mol. wt of about 13 000.