A watery solution of the extractives, etc., of lean meat (usually beef) forms the basis of several nutrient media. This solution is termed "meat extract" and it has been determined empirically that its preparation shall be carried out by extracting half a kilo of moist meat with one litre of water. For many purposes, however, it is more convenient to have a more concentrated extract; one kilo of meat should therefore be extracted with one litre of water, to form "Double Strength" meat extract. It was customary at one time, and is even now in some laboratories to use either "shin of beef" or "beef-steak"--both contain muscle sugar which often needs to be removed before the nutrient medium can be completed. Heart muscle (bullock's heart or sheep's heart) is much to be preferred and from the point of economy, ease and cleanliness of manipulation, and extractive value, the imported frozen bullock's hearts provide the best extract. Meat extract (Fleischwasser) is prepared as follows: 1. Measure 1000 c.c. of distilled water into a large flask (or glass beaker, or enamelled iron pot) and add 1000 grammes (roughly, 2-1/2 pounds) of fresh lean meat--e. g., bullock's heart--finely minced in a mincing machine. 2. Heat the mixture gently in a water-bath, taking care that the temperature of the contents of the flask does not exceed 40° C. for the first twenty minutes. (This dissolves out the soluble proteids, extractives, salts, etc.) 3. Now raise the temperature of the mixture to the boiling-point, and maintain at this temperature for ten minutes. (This precipitates some of the albumins, the hæmoglobin, etc., from the solution.) 4. Strain the mixture through sterile butter muslin or a perforated porcelain funnel, then filter the liquid through Swedish filter paper into a sterile "normal" litre flask, and when cold make up to 1000 c.c. by the addition of distilled water--to replace the loss from evaporation. 5. If not needed at once, sterilise the meat extract in bulk in the steam steriliser for twenty minutes on each of three consecutive days. Calf, sheep, or chicken flesh is occasionally substituted for the beef; or the meat extract may be prepared from animal viscera, such as brain, spleen, liver, or kidneys. NOTE.--As an alternative method, 5 c.c. of Brand's meat juice or 3 grammes of Wyeth's beef juice, or 10 grammes Liebig's extract of meat (Lemco) may be dissolved in 1000 c.c. distilled water, and heated and filtered as above to form ordinary or single strength meat extract. Media, prepared from such meat extracts are, however, eminently unsatisfactory when used for the cultivation of the more highly parasitic bacteria; although when working in tropical and subtropical regions their use is well-nigh compulsory.