A hemagglutination test has been developed for P. pestis. The antigen is contained in old broth cultures, in extracts of killed and dried organisms and in alcohol-precipitated envelope antigens, but not in the protein fractions soluble in 0.25 saturation of ammonium sulfate solution or in those isolated by precipitation of soluble extracts by 0.3 or more saturated ammonium sulfate. The crude antigen has a low nitrogen content and is likely to be a polysaccharide. The use of the hemagglutination test or of the inhibition of hemagglutination may be valuable in ascertaining the identity of plague antigens. The hemagglutination antigen appears not to play an important role as a protective antigen in plague.