The cross‐reactivity of O antigens prepared fromBacteroides fragilis, otherBacteroidesspecies and fromFusobacteriumhas been examined by indirect haemagglutination and inhibition of haemagglutination. Fifteen of 20B. fragilisss.fragilisstrains showed O‐antigenic cross‐reactivity with one or more of the test strains ofB. fragilisss.fragilis:NCTC 9343, Lille E 323 and SBL B55. The same applies also to 3 strains classified asB. coagulans, B. hypermegasandB. putredinis.The multispecificity ofB. fragilisO antigens is pronounced. Test systems for demonstration of 9 specificities, all harboured by one or more of the 3 test strains, have been worke