During a systemic investigation into O-antigenic relations within the family Enterobacteriaceae, with special emphasis on serological relationships between O-antigens (or O-groups) of E. coli and various O-groupps of Salmonella, Arizona, and Citrobacter, our tests confirmed some findings previously descried by other authors, some were not confirmed, and other antigenic relationships were discovered which have not yet been published. Individually tested were O-Antigens of 142 serologically defined E. coli types by using O-factorsera of Salmonella, Arizona, and Citrobacter for the slide agglutination. The slide agglutination was applied in preference to the tube-agglutination, not only because the former has been generally adopted but it also shows broader reactions...