This study concerns the preparation of reagents for identifying the somatic O antigens of Salmonella enteritidis. Coagglutination reagents (COAGs) with antibody fixed to killed and stabilized protein A-bearing staphylococci were prepared with antisera which were used for identifying the somatic O antigens of S. enteritidis by the slide agglutination test. The reactions of the COAGs were compared with those obtained with the grouping antisera in routine slide agglutination tests in which 41 or more serologically different Salmonella strains, representing most of the known groups, were used. One-third of the COAGs gave identical reactions to those of the slide agglutination antisera. The reactions of the other COAGs varied from the slide agglutination antisera results, some by many reactions and others by only a few. The coagglutination procedure was more reactive than the routine slide agglutination test and resulted in cross-reactions which were not observed in the original grouping antisera. More COAGs were specific when they were tested with alcohol-treated cultures than with live cultures. Coagglutination conserves antiserum, allowing about 12 times as many tests for a given volume of group-specific glycerolized antiserum as does the slide agglutination method.