It has been noted in the literature that certain salts enter into specific interaction with proteins. As a result of this, they may act as salting-in agents. We have investigated the effect of magnesium chloride which is known to possess such unusual properties on the retention of proteins in hydrophobic-interaction chromatography. First the retention behaviour of amino acids and small peptides having a wide polarity range was studied on reversed-phase columns using eluents containing (NH4)2SO4, MgSO4 or MgCl2, in wide the concentration ranges. For less polar eluites plots of the logarithmic retention factors against the salt concentration were found to be linear, whereas the more polar species showed irregular behavior. The retention of a wide range of proteins was measured on a TSK Phenyl-5-PW column using eluents containing (NH4)2SO4, MgSO4 or MgCl2 at different concentrations.