The immunoperoxidase technique together with electron microscopy has been examined for the ultrastructural localization of staphylococcal protein A, the immunoferritin method being included for comparison. The results show that protein A is uniformly distributed in the whole cell wall. Both the direct and indirect methods, Fab‐ as well as Fc‐reactions, showed identical results. The immunoperoxidase method was superior to the immunoferritin method, especially when applied to thin sections of the bacteria, and the clear specificity demonstrated indicates a useful method for localization of cellular antigens.

Mucopeptides from one Staphylococcus aureus, two Staph. epidermidis and two Micrococcus strains have been prepared and examined for chemical composition and for serological activity. The chemical analyses showed differences in the mucopeptide structures, especially between staphylococci and micrococci, some differences appearing to be related to variations in the inter‐peptide bridges. No precipitation was observed, but agglutination was obtained in antisera to whole bacteria as well as in antisera to mucopeptides. Cross‐absorption studies showed the presence of shared and specific antigenic determinants. None of the mucopeptides sensitized erythrocytes for agglutination in antisera, but the staphylococcal mucopeptides, in contrast to the micrococcal, induced the production of haemagglutinins in rabbits. Antibodies to mucopeptides were found in human serum and colostrum. The sera contained antibodies of the IgG class; colostrum mostly IgA antibodies. Sera with increased anti‐staphyiolysin titres contained higher amounts of IgG than normal sera. Fab but not Fc fragments of IgG were found to bind to mucopeptide, and complexes of mucopeptide and IgG fixed complement. Injection of mucopeptides into rabbit and human skin provoked dermonestronic reactions.

Antigenic determinants of Staphylococcus aureus mucopeptide and corresponding antibodies in rabbit antiserum have been studied, using synthetic peptides in inhibition of indirect haemagglutination and immunoadsorbent techniques. Results are presented to show that at least 3 determinants are present in the peptide subunits. The pentapeptide L‐Ala‐γ‐D‐Glu‐L‐Lys‐D‐Ala‐D‐Ala exhibits 2 determinants, one of which is located to the C‐terminal D‐Ala‐D‐Ala, and the penta‐glycine bridge one determinant.

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Human serum was applied to formalinized parasites (RH strain) which had been spotted on glass slides. Peroxidase‐labelled rabbit antihuman gammaglobulin and then a diamino‐benzidine‐H2O2 solution were added separately. The brown color of the parasites, resulting from the enzyme‐substrate reaction, was observed with a light microscope. Results of the immunoperoxidase test were compared with those of the methylene blue dye and indirect hemagglutination tests with excellent agreement of titers being obtained with the dye test.