1. Of 1552 strains of Staph. pyogenes tested by the slide-agglutination technique, using the absorbed and unabsorbed sera suggested by Cowan, 74% of strains were identified while 26% remained untyped. Of untyped strains, half were inagglutinable while the remainder gave ‘indefinite’ results and remained unclassified. The introduction of two new sera, used diluted but unabsorbed, considerably reduced the number of untyped organisms but made little difference to the proportion of ‘indefinite’ strains. The method was simple and quick, but the variety of cross-reactions often prevented a clearcut differentiation of strains and the number of types identifiable was limited.

2. Of three methods of agglutination, titrations on glass slides were preferred because by this technique end-points were clearer than those obtained in a water-bath or by the method for determination of Vi antibody.

3. Agglutination of seventeen strains of Staph. pyogenes with homologous and heterologous sera showed that the use of unabsorbed sera was of limited value for typing because of the marked cross-reactions.

4. Christie & Keogh's division of Staph. pyogenes into nine types was promising but their absorbing methods allowed much antigenic overlap.

5. Experiments with absorbed sera showed that no single strain of Staph. pyogenes could be used successfully to absorb all sera. There appeared to be groups or pairs of types in which serologically the individual members resembled each other very closely and yet were not identical. In these instances the absorption of a serum by a heterologous strain nearly related to the homologous strain proved of most value in the preparation of specific sera.

6. A set of seventeen absorbed sera was prepared from Cowan's three types of Staph. pyogenes, from six of Christie & Keogh's type strains and from eight additional strains freshly isolated from staphylococcal lesions. With some absorbed sera there was specific agglutination with the homologous strain and no heterologous reactions, other sera reacted with several strains and could not be made specific.

7. Seven specific serological types were recognized while a further eight types showed less specific reactions and were identifiable by a pattern of reactions. A number of strains differed slightly one from another in their agglutination reactions, and it was not possible to decide whether these were additional types or variants within types.

8. In a small series of experiments the precipitin reaction, using unabsorbed sera, seemed of little value in the differentiation of strains of Staph. pyogenes.

9. Of 259 coagulase-positive strains examined during 1946 and 1947, all were identified by agglutination reactions, although they varied in their specificity; 67.5% of these strains were identified by staphylococcal bacteriophage reactions.

10. A preliminary comparison of serological and bacteriophage types showed a fairly regular but not complete correlation between the results obtained by the two methods. A number of serological types were regularly lysed by the same bacteriophage filtrates; a few were consistently untypable by phage indicating that filtrates had not as yet been found for them or that the strains were phage resistant.

Others which were not clear-cut in their reactions and which were recognized by patterns of reactions with both sera and phage filtrates did not always show a close correlation in the typing results.

11. The application of both serological and bacteriophage typing to the investigation of outbreaks of staphylococcal infection has proved to be of value in tracing sources and paths of spread of infection. In several instances the use of both methods of typing has led to conclusions which would not have been warranted if one method alone had been used.