Hen egg white lysozyme has been succinylated, the isoelectric point being shifted thereby from about 11 to 4.5-4.7. Its adsorption on a chromatography grade hydroxyapatite has been investigated at 20 degrees C and in the pH range from 5.9 to 7.4. Adsorption takes place despite an electrical repulsion between the surface and the adsorbate; consequently, it is favored by a decrease of pH and phosphate concentration and an increase of calcium concentration and ionic strength. While adsorption of native lysozyme is driven by electrostatic attraction to the surface, adsorption of succinylated lysozyme is controlled by hydrophobic interactions. This may be attributed to a different structure of modified lysozyme, compared to native lysozyme, or to a greater tendency to undergo conformational changes.