Electron paramagnetic resonance (EPR) spectroscopy has been investigated as a method for exploring the transient development of fouling in single protein ultrafiltration and as a potential method to determine protein-membrane interaction. Preliminary research showed that EPR was an ideal method for analyzing short-time solute uptake in ultrafiltration. Studies were performed using spin-labeled BSA (65,000 Da) in solutions of pH 5 or 7, and a total salt concentration of 0.05 or 0.15 M. Polysulfone membranes of 100,000 MWCO were used. The results showed that a substantial amount of protein uptake by the membrane occurred within the first few minutes of ultrafiltration. This implies that pore plugging is a significant step in the overall irreversible adsorption of proteins on and within membranes. Double spin-labeled lysozyme (15,000 Da) was ultrafiltered with 10,000 and 30,000 MWCO polysulfone membranes and the EPR spectra were analyzed. The results showed that the proteins on the membrane surface may have a different conformation than the proteins trapped and lodged in the membrane pores. Preliminary analysis indicates that the proteins within the pores may be more compressed than those on the surface. The resulting spectra did not represent conformations observed for highly constricted proteins or denatured proteins.