In nearly all chemical industries the problem of wastes is one of importance, and in any scientific study of waste disposal, the concept of the B.O.D. of the waste is vital. The B.O.D. is usually measured by an arbitrary standard test procedure, but may be determined by manometric measurement of the free oxygen utilized. In either case, the B.O.D. determination requires at least 24 hours. Since the free oxygen involved in the B.O.D. of a waste is utilized in metabolic processes of various microorganisms present, an increase in the number of microorganisms lowers the time required for oxidation of the wastes. In this investigation, the B.O.D. of several wastes was determined by measuring the change in the oxygen uptake of high concentrations of microorganisms because of the presence of the waste. These determinations involved a modification of the resting cell technic used in conjunction with direct Warburg technics. High concentrations of washed cells were prepared and small amounts of the waste added. A control was prepared with distilled water. The effects of waste concentration, bacterial concentration, and temperature were studied. The wastes studied included raw sewage, blowdown liquor and total mill wastes from a semichemical pulp mill, and effluent from an anaerobic, sewage-blowdown liquor digester. The concentrations of microorganisms used ranged from 3.4 to 17.0 milligrams of dry bacterial cells in a total volume of 2.5 milliliters of a 0.05 molar phosphate buffer at a pH of 6.8. The manometric B.O.D. was determined at 30°C. The manometric B.O.D. remained constant at 1,785 parts per million for 1:50 and 1:125 volumetric dilutions of the effluent. The manometric B.O.D. remained constant at 10,200 parts per million for volumetric dilutions ranging from 1:125 to 1:1000 for a sample of the blowdown liquor. The manometric B.O.D. of sewage, blowdown liquor, and mixtures of the two wastes was determined. The values obtained were compared with the standard five-day B.O.D. The manometric B.O.D. of the sewage and the blowdown liquor was 413 parts per million and 13,760 parts per million, respectively. The standard five-day B.O.D. of the sewage and the blowdown liquor was 495 parts per million and 37,800 parts per million, respectively. The manometric B.O.D. of 1:1 by volume mixture of the two wastes was 9,900 parts per million, while the standard five-day B.O.D. was 31,200 parts per million. / Master of Science