A porcelain plate microtest by which the decarboxylation of L-histidine by Clostridia could be detected within 6 hours is described. It was more sensitive than chromatographic procedures for detecting histamine production, using ninhydrin or Pauly’s reagent as a developing agent, which were used for confirmation. All of 26 type A strains of Clostridium perfringens and the one type F strain tested decarboxylated histidine. Strongly positive tests were obtained except for some of the Hobbs type strains that gave much weaker reactions. Two type C strains gave negative tests. Fifteen of 19 strains of Clostridium bifermentans also produced decarboxylation, but the reactions were weak and inconsistent. No strains of 18 other species of Clostridia tested produced decarboxylation.