We report a. flow-injection biosensor system with a capacitive transducer for assay and quality control of human immunoglobulin G (hIgG). The sensing platform is based on self-assembled monolayers (SAMs) of carboxylic acid terminated alkyl-thiols with covalently attached concanavalin A. The electrochemical characteristics of the sensor surface were assessed by cyclic voltammetry using a permeable redox couple (potassium ferricyanide). The developed biosensor proved capable of performing a sensitive label-free assay of hIgG with a detection limit of. 1.0 mu g mL(-1). The capacitance response depended linearly on hIgG concentration over the range from 5.0 to 100 mu g mL(-1), in a logarithmic plot. Typical measurements were performed in 15 min and up to 18 successive assays were achieved without significant loss of sensitivity: using a single electrode. In addition, the biosensor can detect hIgG aggregates with concentrations as low as 0.01%. of the total hIgG content (5.0 mu g mL(-1)). Hence, it represents a potential post-size-exclusion chromatography-UV (post-SEC-UV) binding assay for in-process quality control of hIgG, which cannot be detected by SEC-UV singly at concentrations below 0.3% of the total hIgG content. Biotechnol. Bioeng. 2009;104: 312-320. (C) 2009 Wiley Periodicals, Inc.