ABSTRACT The objective of this study was to characterize raltegravir (RAL) binding to albumin and alpha-1-acid glycoprotein (AAG). Unbound and bound RAL were separated by ultrafiltration. The association constant ( K a ) was estimated by a graphical method. In HIV-infected patients, the average plasma protein binding is 76%. RAL did not bind to AAG but bound to nonsaturable, low-affinity albumin sites with an n (number of sites) · K a product of 9.8 × 10 2 liters/mol. A pH increase of 0.2 U led to a 2% increase in the bound fraction.