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SUMMARY. A p o r c e l a i n p l a t e m i c r o t e s t by w h i c h t h e d e c a r b a x y l a t i o n o f L -h i s t i d i n e b y c l o st r i d i a c o u l d b e d e t e c t e d w i t h i n 6 h o u r s i s d e -s c r i b e d . I t w a s m o r e s e n s i t i v e t h a n c h r o m at o g r a p h i c p r o c e d u r e s f o r d e t e c t i n g h i s t a m i n e p r o d u c t i o n , u s i n g n i n h y d r i n o r P a u l y ' s r ea g e n t a s a d e v e l o p i n g a g e n t , w h i c h w e r e u s e d f o r c o n f i r m a t i o n . A l l of 2 6 t y p e A s t r a i n s o f C l o s t r i d i u m p e r f r i n g e n s a n d t h e o n e t y p e F s t r a i n t e s t e d d e c a r b o x y l a t e d h i s t i d i n e . S t r o n g l y p o s i t i v e t e s t s w e r e o b t a i n e d e x c e p t f o r s o m e o f t h e H o b b s t y p e s t r a i n s t h a t g a v e m u c h w e a k e r r e a c t i o n s .T w o t y p e C s t r a i n s g a v e n e g a t i v e t e s t s .F i f t e e n o f 1 9 s t r a i n s o f C l o s ' t r i d i u m b i f e r m e n t a n s a l s o p r o d u c e d d e c a r b o x y l a t i o n , b u t t h e r e a c t i o n s w e r e w e a k a n d i n c o n s i s t e n t . N o s t r a i n s of 1 8 o t h e r s p ec i e s o f c l o s t r i d i a t e s t e d p r o d u c e d d e c a r b o x yl a t i o n .Gale (1941 , 1946) reported that 9 of 10 s t r a i n s of Clostridium perfringens, type A, as well as s t r a i n s of types Bj C, and D, decarboxylated histidine; i t was a l s o reported t h a t s o m e s t r a i n s of ,C. biferrnentans and ,C. fallax decarboxylated i t m o r e slowly. H e detected decarboxylase production by the evolution of carbon dioxide by washed-cell suspensions of o r g a n i s m s i n a manometer.We have t e s t e d a m o r e extensive s e r i e s of s t r a i n s of clostridia to d e t e r m i n e the taxonomic significance of histidine -de car boxy la s e production and have investigated simpliThis work w a s supported by N S F Undergraduate R e s e a r c h Participation grant GE-4148 and Public Health Service Res e a r c h Grant A1 04958 f r o m the National Institute of Allergy and Infectious Diseases.