Monoclonal antibodies produced against protein-type inhibitors from pea or soybean were used for investigations of the chemical and antigenic properties of trypsin and chymotrypsin inhibitors from pea. Cross-reactivity studies revealed only the existence of Bowman-Birk-type inhibitors in pea. The inhibitors could be divided into at least two groups of iso-inhibitors based on their characteristics in binding to different monoclonal antibodies produced against pea trypsin inhibitors or Bowman-Birk inhibitor from soybean. The inhibitor contents in a series of pea extracts were measured in an ELISA-based system using the different antibodies. Comparison with the inhibitor activity measured by traditional enzymatic analysis performed as microassay showed that only the inhibitor content of one of the two inhibitor groups correlated strongly with the trypsin inhibitor activity. The immunoassay was also shown to be suitable for measurement of inhibitor content after heat treatment as the results suggested that the two inhibitor groups contained equally heat resistant inhibitors.