A method based on capillary electrophoresis (CE) with electrochemical (EC) detection for the determination of both total homocysteine (tHcy) and protein-bound homocysteine (pbHcy) in plasma is described, Both end-column and off-column ampero-metric detection were investigated. Off-column detection resulted in a more sensitive assay for the determination of homocysteine (Hcy). The detection limit for homocysteine was 600 nM using off-column EC detection and the response was linear over the range 1-100 muM. Therefore, this assay is appropriate for the quantification of Hcy over the physiological concentration ranges found in all disease states. Methodologies for the determination of tHcy and pbHcy in human plasma were investigated and optimized and the concentrations of both pbHcy and tHcy in plasma obtained from a healthy individual were determined to be 2.79 +/- 0.31 muM (n = 4) and 3.37 +/- 0.15 muM (n = 3), respectively. The methodology was then transferred to a microchip CE-EC format and Hcy and reduced glutathione (GSH) were detected, Future work will focus on the development of ancillary methodologies to identify the other forms of Hcy in vivo.