This paper presents an imaging capillary isoelectric focusing (CIEF) assay for the determination of the identity, stability, and isoform distribution of a murine monoclonal antibody (MU-B3). The experiments were conducted using a Convergent Bioscience iCE280 instrument. The optimum carrier ampholyte composition that gave the best peak separation was found to be 25% Pharmalyte pH 3-10 and 75% Pharmalyte pH 5-8. The antibody gave a highly reproducible CIEF profile with three major peaks having average isoelectric point (pl) values of 6.83, 6.99, and 7.11. Intraday and interday reproducibility of pl values was found to be within RSD of 0.5%. The CIEF profile was also the same, with an alternate column cartridge and alternate batches of methyl cellulose. A plot of peak areas versus MU-B3 concentration was linear (R-2 = 0.995) up to a concentration of 0.5 mg/mL in the sample solution. Peak area measurements were reproducible to within 7% RSD. The CIEF profiles of two other antibodies were distinctly different from the profile of MU-B3, showing that the assay is specific. After a sample of MU-B3 was subjected to heat stress by exposure to heat at 55degreesC for 4 h, its CIEF profile was altered with extra peaks appearing at lower pl values, indicating that the assay could be used to monitor stability. The result of the heat stress experiment was also confirmed with a parallel slab-gel IEF analysis of the antibody sample before,and after application of the heat stress. The results of this work suggest that imaging CIEF can be used for product testing under a quality control environment. The assay can be used for pl profiling of proteins and for monitoring structural changes (deamidation, glycosylation, etc.) during the manufacturing process and upon storage.