1. =Actinomyces.= Alcohol and formol fixation are best. Good preparations can be obtained with hæmatoxylin and eosin, Van Gieson’s or Weigert-Gram’s. The special staining methods advised give no better results than these simpler stains. Differential staining of clubs and mycelium may be obtained by _Mallory’s method_:— 1. Stain lightly in alum-cochineal. (Powdered cochineal 6 grms., ammonia alum 6 grms., water 100 cc. Boil half an hour, add water lost by evaporation, filter, add crystals of thymol.) 2. Saturated watery eosin 10 minutes. 3. Wash in water. 4. Stain in aniline gentian-violet 2-5 minutes. 5. Wash in physiologic saline solution. 6. Transfer sections to Lugol’s for 1 minute. 7. Pass rapidly through water. 8. Dry thoroughly between folds of filter-paper. 9. Cover section with aniline-oil until clear. 10. Xylol; balsam. Clubs pink; mycelium blue. 2. =Nocardia, Cladothrix, Streptothrix and Leptothrix.= Löffler’s methylene-blue and carbol-fuchsin give good results. The Nocardiæ are acid-fast with dilute acids. They give good preparations with Weigert’s fibrin stain and lithium-carmine. _d. Vibrios._ 1. =Cholera Vibrios.= Gram-negative. Sections may be stained with fuchsin or methylene-blue. _e. Spirilla and Spirochætes._ 1. =Spirillum of Recurrent Fever.= Stain in sections with Levaditi’s silver-method, or with Nikiforoff’s method:— 1. Fix for 24 hours in equal parts of a 5 per cent water solution of potassium bichromate and a saturated solution of mercuric chloride in 0.6 per cent sodium chloride. 2. After-harden in graded alcohols in the incubator. 3. Imbed in paraffin. 4. Stain 24 hours in a mixture of alcoholic 1 per cent solution of tropæolin 5 cc., concentrated watery methylene-blue solution 10 cc., caustic potash solution (1:1000) 2 drops. 5. Wash in water. 6. Dip several times in a mixture of equal parts of absolute alcohol and ether. 7. Oil of bergamot; xylol; balsam. The spirillum of _African relapsing fever_ stains with the same stains as the spirillum Obermeieri. The _spirochætes of Vincent’s angina_ and _fowl-spirillosis_, and the _spirochæte refringens_ stain with watery aniline dyes and with Giemsa’s stain; in section they are stained by the Levaditi method.