2. Cleanse the skin of the distal phalanx of the second finger of the left hand above the root of the nail with lint and ether. Wind the rubber tubing tightly round the second phalanx; puncture with a sterile Hagedorn needle through the cleansed area of skin. 3. Take up a sufficiency of the issuing blood (more or less according to the number of tests to be performed) with a teat pipette, transfer it to the tube of citrate solution and mix thoroughly. Make a second mark on the tube at the upper level of the mixed citrate solution and blood. 4. Place the tube in the centrifuge, counterpoise accurately and centrifugalise until the blood cells are thrown down in a compact mass occupying approximately the same volume as is included between the two pencil marks. The column of fluid in the tube now shows clear supernatant fluid (citrate solution and blood plasma) separated from the sharp cut upper surface of the red deposit of corpuscles by a narrow greyish layer of leucocytes. 5. Remove the supernatant column of citrate solution by means of a teat pipette, fill normal saline solution into the tube up to the upper pencil mark, and distribute the blood cells throughout the saline by means of the teat pipette. Centrifugalise as before. 6. Again remove the supernatant fluid and fill in a fresh supply of saline solution and centrifugalise once more. 7. Remove the supernatant saline solution as nearly down to the level of the leucocytes as can be safely done without removing any of the leucocytes. 8. Next distribute the leucocytes evenly throughout the mass of red cells by rotating the tube between the palms of the hands--just as is done with a tube of liquefied medium prior to pouring a plate. 9. Set the tube upright in the plasticine block near to one end. _Bacterial Emulsion._-- 1. Take an 18- to 24-hour culture of the required bacterium (e. g., Diplococcus pneumoniæ) grown upon sloped blood agar at 37° C. Pour over the surface of the medium some 5 c.c. of normal saline solution. 2. With a platinum loop emulsify the growth from the surface of the medium as evenly as possible in the saline solution. 3. Allow the tube to stand for a few minutes so that the large masses of growth may settle down; transfer the upper portion of the saline suspension to a centrifuge tube and centrifugalise thoroughly. 4. Examine a drop of the supernatant opalescent emulsion microscopically to determine its freedom from clumps and masses. If unsatisfactory prepare another emulsion, this time scraping up the surface growth with a platinum spatula, transferring it to an agate mortar and grinding it up with successive small quantities of normal saline. If satisfactory insert the tube in the plasticine block next to that containing the washed cells. ~Specific Serum.~-- ~Pooled Serum.~--