Development of the field of environmental engineering has long been impeded by the lack of a quick, accurate method of measuring the biochemically oxidizable organic content of waste waters. Such a test is important to the detection and prevention of pollution of rivers, lakes, and other receiving waters. Proper operation of existing waste water treatment plants and the design of new waste treatment plants also depend on such a parameter. Rapid development of the chemical industry in recent years has further emphasized the need for such a test. A new test is presented which measures the disappearance of soluble organics from a substrate in contact with an active mass culture of mixed microbial organisms. Soluble organics in the system are measured by the Chemical Oxygen Demand (COD) test and reported in oxygen equivalents. The system substrate COD reaches a minimum when the soluble organics available to the mass culture for metabolism have been depleted. Substrate COD remains relatively constant from this point on. The difference between the initial substrate COD and the minimum substrate COD reached after aeration is then the Total Biological Oxygen Demand (TbOD) of the system. This TbOD test may be completed in 6 to 8 hours (approximately 4 to 6 hours laboratory time). In addition to measuring the TbOD, the test may be used, with modifications, to define process kinetics for design, to serve as a daily log of plant operation, or be used as a quick method of diagnosing plant operation difficulties. Delineation of plant cyclic load variations is another use for this test. This TbOD test will be particularly useful for the control of operating plants since an acclimated culture is available from the plant at all times.