A study of the effects of a log-phase seeding technique and of the oxidative assimilation relations, in soluble substrates, of mixed cultures of bacteria and protozoa, as found in a typical domestic waste water, was conducted in the Environmental Engineering Laboratory of the Rice University during the 1961-1962 academic year. These studies were part of a broad investigation of the study of the Biochemical Oxygen Demand Progression in soluble substrates. This is believed to be the first work using the log-phase seeding technique in shortening the time to the plateau in oxygen utilization and in the formulation of oxidative assimilation relationships in mixed microbial populations. Previous work on the effects of log-phase seeding and microbial assimilations generally utilized pure cultures and heavy innocula of bacteria with short incubation periods. Substrates studied in this investigation were glucose, glutamic acid, a (1:1) mixture of glucose and glutamic acid, aspartic acid, acetic acid, propionic acid, and lactose. Both Warburg and dilution techniques were utilized. The log-phase seeding technique produced a marked reduction in lag for all of the substrates and a reduction in time to reach the plateau for lactose, glutamic acid, aspartic acid, and propionic acid. The general pattern of the progression of biochemical degradation of soluble substrates in shown in graph form. The plateau in oxygen utilization was highly reproducible and was characteristic for a specific substrate. Based on the data obtained, oxidative assimilation equations were developed for the substrates under study. Theoretical plateaus occurred at 41, 40, 39, 44, 50, 52, and 41 per cent of the theoretical oxygen demand in the case of glucose, a (1:1) mixture of glucose and glutamic acid, glutamic acid, aspartic acid, acetic acid, propionic acid, and lactose, respectively. The nitrogqp supplied by the standard dilution technique for B.O.D. analysis is adequate for glucose and lactose concentrations only up to 8mg/l. At the concentrations of 12.1 mg/10 utilized in this study, the resulting nitrogen deficiency yielded low values for the plateau in oxygen uptake normally representing completion of synthesis.