Previously, Lin et al. (J, Immunol. Methods 125, 67, 1989; J. Chromatogr, 542, 41, 1991) showed that acid pretreated antibodies (Abs) exhibited a twofold higher antigen-binding capacity than native Abs when immobilized to solid surfaces that have strong nonspecific protein adsorption properties (e.g,, polystyrene and silica). In the present article several experiments were conducted in order to elucidate the adsorption mechanism of acid pretreated antibodies. It was found that acid pretreatment time could be used to control the packing efficiency and orientation of Abs adsorbed to silica surfaces hydrophobicized with dichlorodimethylsilane (DDS), Further, our results suggest that acid pretreatment leads to an increased exposure of hydrophobic regions in the constant fragment of the immunoglobulin. Selective adsorption of these regions produces a preferential orientation in which the antigen-binding fragments are accessible to bulk solution. This results in a higher antigen-binding capacity than observed when native Abs (no acid pretreatment) are immobilized on DDS-treated silica surfaces.