During the last five years, as a result of work on the identification of several thousand enteric organisms, particularly Salmonellae, Shigellae and coliform organisms, certain observations concerning cross-agglutinations within the genus Salmonella and between Salmonellae, Shigellae paradysenteriae (Flexner), colon, paracolon and proteus bacilli were made. At first, these cross-reactions were rather confusing; they were followed up more systematically, and some of the results reported from our laboratory (1–3). An attempt will be made to evaluate our results and correlate our experiments with those of other workers in this field (4–12). It is generally acknowledged that the genus Salmonella comprises a variety of gram-negative bacilli, which are relatively uniform biochemically and antigenically related. The Kauffmann-White schema divides them on grounds of their O-antigenic structure into a number of sub-groups (A, B, C, D, E…). The types within these sub-groups are mainly determined by their flagellar (H) antigen combination.