When cells of a marine pseudomonad washed free of medium components with 0.05 m MgSO(4) were suspended in solutions containing 200-mm concentrations of various salts, there was an immediate increase in optical density (OD), followed by a slow decrease. The decrease following the initial increase, but not the increase itself, could be prevented by omitting K(+) from or by adding metabolic inhibitors to the suspending solution. With NaCl, the initial increase in OD rose to a maximum as the salt concentration was increased to 200 mm and then declined at 500 mm. There was a corresponding decrease in intracellular fluid volume to a minimum at 200-mm NaCl and then a rise. When the increased OD produced by NaCl was maintained, the internal Na(+) and Cl(-) could be shown to have reached essentially the same concentration in the cells as in the medium. Thus, the OD changes could not have been due to osmotic effects. No evidence was obtained of a salt-induced aggregation of nuclear material. The OD of suspensions of isolated cell envelopes increased in response to increases in NaCl concentration in the absence but not in the presence of 0.05 m MgSO(4). The data was interpreted to indicate that the salt-induced increases in OD occurring in suspensions of the cells resulted from an interaction of salts with components of the cell envelope, causing contraction of the envelopes and shrinkage of the cells.