1. The reaction of slowly growing pathogenic bacteria with specific fluorescent antibody provided a means for their rapid identification.
2. Fluorescein isothiocyanate was used to label the globulins of antisera as it had a number of advantages over fluorescein isocyanate.
3. Several other isothiocyanates and a few sulphonyl chlorides tested were inferior to flurescein isothiocyanate as labelling agents. Some of the compounds caused protein precipitation, and others gave a less intense or less distinctive fluorescence than did dluorescein isothiocyanate.
4. The tests were performed by taking impressions of bacterial micro-colonies on glass cover-slips, drying and fixing the preparations, staining them with labelled antiserum, washing in saline and mounting in glycerol saline. The preparations were than examined microscopically under ultra-violet light.
5. Micro-colonies of Br. suis, Past. tularensis and Past. pestis, when treated with homologous labelled antiserum, fluoresced brightly under ultra-violet light and the individual bacteria were sharply defined and stained at the periphery.
6. Micro-colonies of these and other bacteria stained with normal labelled serum, or heterologous labelled antiserum, showed either very dim fluorescence under ultra-violet light, with poor definition of in dividual bacteria, or no fluorescence at all.
7. The technique described permitted specific identification of Br. suis, Past. pestis and Past. tularensis within 20 hr. of inoculation of agar plates with material suspected of containing one of these organisms.
This investigation arose out of a line of work suggested by Dr G. S. Wilson. We should like to thank Dr D. W. Henderson and Major L. H. Kent for providing facilities for the work, Mr E. O. Powell for continual encouragement and supervision of the microscopy, Dr D. W. Henderson, Dr M. C. Lancaster and Dr D. A. L. Davies for supplying antisera, and Mr T. W. Pearce for valuable technical assistance and the photographs.