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SUMMARY: The kinetics of the turbidity changes in bacterial suspensions due to variations in the osmotic pressure of the medium were investigated. The time curve of the turbidity changes followed a monomolecular course, but the velocity constant was strongly dependent on the osmotic pressure. The value of the temperature coefficient (Q 10) of the reaction was close to two. The rapid adjustment of the turbidity (increase or decrease) to the changes in osmotic pressure of the medium was followed by a phase of slow decline in light extinction. In this slow phase the rate of turbidity decrease was independent of the prevailing osmotic pressure, but varied with the nature of the solute used; it was markedly accelerated by KCN or HgCl2. The effect of HgCl2 was annulled by thiol compounds. Certain substances affected the optical effect and the respiration in a parallel manner. The similarity between these observations and the findings of several authors with regard to turbidity changes in mito-chondrial suspensions is discussed.