Polymer microbeads are witnessing renewed interest for performing biomolecule recognition assays with distinct advantages over planar microarray technology. In this study, DNA hybridization assays are performed on the surfaces of 1-microm-diameter, synthetically modified polystyrene microbeads. The microbead surfaces contain varying amounts of poly(acrylic acid) as a source of carboxylate groups to which a DNA capture strand may bind. Through a series of controlled experiments in which the microbead carboxylate density and DNA:surface area ratios are systematically altered, we find that the density of carboxylate groups on the microbead surface may be the most important parameter affecting not only the total number of DNA strands that may bind to the microbead surface but, surprisingly, also the efficiency of DNA hybridization with complementary strands. These studies are aimed directly at understanding the physical interactions between DNA strands and an anionic microbead surface.