The oxidation of antibody carbohydrate residues is a common approach used for site-specific antibody immobilization or modification. In this study a flow injection analysis system (FIA) was developed for monitoring antibody oxidation. Antibodies were oxidized with periodate and the resulting aldehyde groups were labeled with Lucifer yellow (\mathrm{CH}) (LyCH). The labeled antibodies were then injected onto an FIA system where the amount of (\mathrm{LyCH}) label was determined by absorbance measurements at (428 \mathrm{~nm}) and the amount of antibody was determined using an on-line bicinchoninic acid protein assay. The analysis time was (2 \mathrm{~min}) per (20 \mu \mathrm{l}) sample injection. The limits of detection for rabbit immunoglobulin G (IgG) and LyCH were (1 \times 10^{-8}) and (4 \times 10^{-7}) (M), respectively. The dynamic ranges for IgG and LyCH extended to (2 \times 10^{-5}) and (7 \times 10^{-3} \mathrm{M}). The within-run precision was (\pm 5 %) or less for both analytes. Studies with known LyCH/antibody mixtures indicated that the FIA system had greater accuracy than manual methods at high (\mathrm{LyCH}) levels. One specific application studied for this system was its use in monitoring the time course of periodate-antibody oxidation. c 1994 Academic Press, Inc.