Whey samples were screened for the presence of the proteinase inhibitor α2-macroglobulin (α2M). From an enzymic test, α2M levels in normal whey varied in the range 0·49–0·84% of the serum level, whereas in mastitis whey the activity was markedly increased, reaching values between 0·91 and 138·5% (median 7·2%) of standard serum level. In mastitis milk samples but not in normal milk α2M was also detected by double immunodiffusion and Western blotting. The proteinase inhibitor was purified from a mastitis milk sample with high α2M activity (138·5% of serum level). In SDS-PAGE, native-PAGE and in double immunodiffusion analysis the inhibitor appeared indistinguishable from plasma-derived α2M. The α2M preparation from mastitis whey migrated essentially as native α2M, representing the ‘slow’ form of the molecule. Treatment with trypsin transformed the α2M preparation to the electrophoretic ‘fast’ form, but treatment with methylamine had only a minor effect. The receptor recognition sites were not exposed on the isolated α2M molecule but could be readily exposed by treatment of the proteinase inhibitor with trypsin, which further proved that the isolated α2M was in the entire native, functionally active state.