A study of the effects of the chemical dimethyl sulfoxide in an aerobic, mixed culture, single component substrate system was undertaken in the Environmental Engineering. Laboratories at Rice University. This study represents a portion of a broader project aimed at elucidation of the basic processes involved in substrate utilization by a biological system and the evaluation and significance of the concomitant oxygen demand exerted by such a system. By observation of changes induced in a well defined system by a biologically active substance such as DMSO, it was hoped to discover something about the mechanism of action of DMSO. The chemical's remarkable ability to increase permeability of living membranes is a quality that could help determine the influence of the bacterial membrane on the removal rate of organics from solution. The work showed that DMSO had effects on all systems Studied. Because additional effects were observed in systems operating under nitrogen limited conditions, it was necessary to define the effects of nitrogen limitation in all systems under consideration. The discovery was made that limiting the nitrogen in these systems resulted in a stimulation of oxidation with a concomitant increase in the oxygen uptake up to a point of approximately fifty percent of the required nitrogen. Past this point the synthesis of new organisms was inhibited to the extent that oxygen uptake was described by a straight line at a much reduced rate from the control. Thus, by the time the point was reached where a plateau would normally have occurred, secondary oxygen uptake by predators was well established, and the plateau phase of the oxygen uptake curve was obscured or eliminated. Introduction of DSO into a system operating under conditions of nitrogen limitation was observed to have several effects. There was an increase in the oxygen uptake value measured at 50 hours in those nitrogen limited systems. There was also an increase in rate of oxygen uptake in severely limited systems (40% or less of required nitrogen). In addition, DMSO was observed to have the following effects in both nitrogen limited and nitrogen sufficient systems. There was a lag time before onset of maximum oxygen uptake rate that increased as the concentration of DMS0 increased. An inhibition of predator activity as exhibited by the delay of the onset of secondary oxygen uptake past the plateau was observed. At a given degree of nitrogen limitation, no effect of increasing DMS0 concentration could be shown other than those effects observed in non-limited systems.