The presence of a chymotrypsin-like enzyme in rat mast cells has been confirmed, and it has been shown that intact mast cells contain the fully activated enzyme without losing their histamine content. It has been shown by means of inhibitors that the chymotrypsin-like enzyme is distinct from the heat-labile SH enzyme required for the histamine releasing action of compound 48/80. The synthetic ester used as a substrate for chymotrypsin released histamine from rat mast cells probably by a non-enzymic reaction.