S. aureus aqueous extract and K. pneumoniae hydroxylamine vaccine were studied by means of chemical and immunochemical analytical techniques. The preparations were found to contain, respectively, 7.0% nad 53.5% of neutral monosaccharides, 6.5% and 0.7% of nucleic acids, as well as protein in approximately equal amounts (11.63-14.0%). In experiment of immunodiffusion, immunoelectrophoresis and rocket immunoelectrophoresis in homologous systems with hyperimmune antimicrobial sera the preparations were characterized by serological heterogeneity. After their combination with Escherichia coli aqueous extract and Proteus hydroxylamine preparation their serological characteristics remaIned unchanged. The study of cross reactions of all components of the combined preparations with hyperimmune rabbit sera to the corresponding microorganisms revealed that only Klebsiella component of the combined vaccine reacted with all hyperimmune sera. The preparation of Proteus showed the lowest activity, it reacted only with hyperimmune sera to K. pneumoniae. Besides, no reaction of S. aureus component with sera to E. coli and no reaction of the preparation of E. coli with antistaphylococcal serum were observed.