Capillary reversed-phase liquid chromatography (RPLC) was coupled on-line to competitive capillary electrophoresis immunoassay (CEIA) to improve concentration sensitivity of the competitive CEIA and to provide a means for detecting multiple species that cross-react with antibody. A competitive CEIA for glucagon was used for demonstration of this technique. Five-microliter samples were injected onto a 4-cm-long by 50-micron-i.d. RPLC column. Sample was desorbed by gradient elution, mixed on-line with fluorescently labeled glucagon and anti-glucagon, incubated in a continuous-flow reaction capillary, and analyzed by capillary electrophoresis with flow-gated injection and laser-induced fluorescence detection. Electrophoretic analysis of the reactor stream was performed every 1.5 s, allowing nearly continuous monitoring of the RPLC separation. Preconcentration achieved by RPLC allowed improvement in the detection limit from 760 to 20 pM. Addition of the RPLC column also allowed multiple cross-reactive species to be differentiated by first separating them chromatographically and then detecting them with the immunoassay. The technique was used to measure glucagon secretion from single islets of Langerhans and to differentiate cross-reactive forms of glucagon with one assay.