Decrease in the protease activity of the culture fluid observed at later stages of the antigen-antibody reaction is believed to be due to the release of an inhibitor by the cells. The inhibitor was submitted to partial purification: it is heat-stable, non-precipitated by trichloracetic acid and non-dialyzable. It inhibits certain cellular and tissue proteases and papain but is inactive against trypsin. It is suggested that the balance between protease and anti-protease released may determine the intensity, extent, and duration of certain sensitization phenomena.

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