IV. STAIN. With the exception of Malta fever and tubercle bacilli, Leishman’s stain, consisting of eosin and methylene-blue, in combination, or Jenner’s stain, is employed. Carbol-fuchsin is employed for the tubercle bacilli. METHOD OF OBTAINING OPSONIC INDEX. Special capillary pipettes, graduated into equal divisions, are required; then by means of a rubber tube or bulb equal quantities of washed corpuscles, bacterial emulsion, and patient’s serum are taken up, and all blown out upon a glass slide and thoroughly mixed. The mixture is then drawn up in a small pipette, sealed, and placed in an incubator of the ordinary type, or preferably in an opsonic incubator as proposed by Freeman; it is heated to 37° to 40° C. for fifteen minutes. A similar tube is prepared, except that the normal serum or “Pool” is employed instead of the patient’s serum. At the end of fifteen minutes the smears on microscopical slides are made from each tube, fixed, and stained. A good field is selected, one containing many leucocytes, the number of germs counted in the first fifty or hundred observed, and an average per leucocyte determined. If the average in the mixture containing the normal serum is two, we would say that the normal opsonic index is two, and if the mixture containing the patient’s blood serum is one per corpuscle, we would say that his index was one-half, etc. Wright and his pupils, as the result of numerous observations, classify diseases due to bacterial infections: (1) “Diseases in which the bacterial process is strictly localized” or “shut off from the lymph and blood circulations.” Furunculosis, lupus, tuberculosis, etc., in fact most chronic infections, belong to this class. “In this class the opsonic index of the blood is persistently below normal, owing to the absence of immunizing stimuli.” (2) “Diseases in which the bacterial process is but loosely shut off, especially from the lymph circulation.” In these, usually acute, infections immunizing products from the invading bacteria from time to time get into the circulation, and the opsonic index may be normal or above or below normal. Good examples of this class are tuberculous joints, etc. (3) “Diseases in which the bacterial infection is in the blood stream.” In this class belong acute infectious diseases, septicemia, endocarditis, Malta fever, etc. In these “the opsonic index is generally below normal.” Wright and Douglas lay down the following general principles for treating cases of bacterial infections: (1) “Isolate in pure culture the causative microörganism.” (2) “Estimate the opsonic power of the patient’s blood to this microörganism.” (3) “If the opsonic index be at or below normal, prepare and standardize a vaccine from this microörganism.” (4) “Inoculate the patient with this vaccine with appropriate doses and at proper intervals, as shown by a systematic estimation of the opsonic content of the patient’s blood.”