1. A methylamine-sensitive inhibitor was present in the plasma of B. glabrata. 2. This inhibitor decreased trypsin activity against a protein substrate, however trypsin retained activity against a low molecular weight substrate in the presence of the inhibitor. 3. Snail plasma protected trypsin from inhibition by soybean trypsin inhibitor. 4. The results give evidence for an alpha-macroglobulin proteinase inhibitor in the plasma of this gastropod mollusc.