Adsorption of biomolecules onto microchannel surfaces remains a critical issue in microfluidic devices. This paper investigates the adsorption of fibrinogen on glass microcapillaries using an immunoassay method (ELISA) and X-ray photoelectron spectroscopy (XPS). Various adsorption conditions such as protein concentrations and incubation times, buffer pH, buffer ionic strengths and effects of flow are presented. ELISA is successfully demonstrated as a facile and robust technique to examine these phenomena. The highest adsorption level occurs near the isoelectric point of fibrinogen (pH 5.0) and low buffer ionic strengths (0-8 mM). Microchannel surface saturation was achieved at a fibrinogen solution concentration of approximately 50 microg ml(-1). Fibrinogen adsorption under flow was always higher than that seen in static systems. The importance of diffusion phenomena in microchannels on protein adsorption was demonstrated. ELISA experiments using fused silica and PEEK have also confirmed significant adsorption on these mass spectrometer transfer line materials.