This article presents the first example of a microfluidic chip for heterogeneous bioassays using a locally immobilized biospecitic layer and operated electrokinetically. The reaction chamber has picoliter dimensions and is integrated into a network of microchannels etched in glass. The high affinity of protein A (PA) for rabbit immunoglobulin G (rIgG) was exploited for chip testing, with PA being immobilized on microchannel walls and fluorescently labeled (CyS) rIgG serving as sample. It was possible to operate the chip in an immunoaffnity chromatographic manner, using electrokinetically pumped solutions. Concentration of antibody from dilute solution onto the solid phase was demonstrated, with signal gains of ∼30 possible. A dose-response curve for Cy5-rIgG was obtained for concentrations down to 50 nM, for an incubation time of 200 s. The flexibility of chip layout was demonstrated for competitive immunoassay of rIgG, using both a combined sample/tracer incubation and sequential addition of these solutions. With assay times generally below 5 rain for this unoptimized device, the microfluidic approach described shows great potential for many high-throughput screening applications.