1. alpha(2)-Macroglobulin is known to bind and inhibit a number of serine proteinases. We show that it binds thiol and carboxyl proteinases, and there is now reason to believe that alpha(2)-macroglobulin can bind essentially all proteinases. 2. Radiochemically labelled trypsin, chymotrypsin, cathepsin B1 and papain are bound by alpha(2)-macroglobulin in an approximately equimolar ratio. Equimolar binding was confirmed for trypsin by activesite titration. 3. Pretreatment of alpha(2)-macroglobulin with a saturating amount of one proteinase prevented the subsequent binding of another. We conclude that each molecule of alpha(2)-macroglobulin is able to react with one molecule of proteinase only. 4. alpha(2)-Macroglobulin did not react with exopeptidases, non-proteolytic hydrolases or inactive forms of endopeptidases. 5. The literature on binding and inhibition of proteinases by alpha(2)-macroglobulin is reviewed, and from consideration of this and our own work several general characteristics of the interaction can be discerned. 6. A model is proposed for the molecular mechanism of the interaction of alpha(2)-macroglobulin with proteinases. It is suggested that the enzyme cleaves a peptide bond in a sensitive region of the macroglobulin, and that this results in a conformational change in the alpha(2)-macroglobulin molecule that traps the enzyme irreversibly. Access of substrates to the active site of the enzyme becomes sterically hindered, causing inhibition that is most pronounced with large substrate molecules. 7. The possible physiological importance of the unique binding characteristics of alpha(2)-macroglobulin is discussed.