In the previous paper, it was reported that the cyanide-activated papain is apparently inhibited by active methylene group and active imide group in barbituric acid. The active methylene group in barbituric acid is derived from malonic acid. Morgan and Friedmann have been shown that maleic acid and SH-compounds interact with the formation of stable addition compounds. They have applied this reaction to the study of some SH-enzyme reaction. The present research has been planed to clarify relation between activity of papain and influence of the dibasic acids.