We have measured the free solution electrophoretic mobility of micron-size lysozyme crystals suspended in aqueous 0.05 M sodium acetate buffer at 10 degreesC to which was added one of the salts, NaCl, NaNO3, or NaSCN. The pH ranged between 3.75 and 5.5 while the ionic strength varied between 0.15 and 0.55 M. After determining the viscosities and densities of these solutions, zeta potentials, which ranged between 2.8 and 16.3 mV and which depended upon pH and ionic strength, could be calculated for these crystals. These positive zeta potentials represent an electrostatic barrier to the growth of a crystal by the addition of individual lysozyme molecules, which in acid solution are macro-ions with high positive charge. Although the electro-hydrodynamic theory of particle electrophoresis predicts that the mobility should be independent of ionic strength under the conditions of our experiment, we observed the mobility to decrease with increasing salt concentration. By assuming that this decrease was due to anion adsorption by the lysozyme crystal, we used the Langmuir adsorption isotherm to predict that the electrostatic barrier to crystallization should go through zero and become attractive at sufficiently high salt concentration.