The salting-out effect of simple electrolytes on lysozyme has been studied by measuring the second virial coefficient B ₂ of the osmotic pressure as a function of salt concentration, and for different salts. The aim of this work has been to find a microscopic counterpart of the empirical Hofmeister series for the efficiency of cations and anions in inducing protein crystallization. The experimental results show that, for large enough ionic strengths, B ₂ scales linearly with the salt concentration. This trend is common to a number of different monovalent salts, however with efficiency strongly dependent on the specific anion. Conversely, changing the cation does not appreciably affect B ₂ . The significance of these findings for the investigation of protein interactions near crystallization is discussed.