A highly sensitive piezoelectric crystal immunosensor has been developed for the detection of human complement C4. Three methods for immobilization of anti-C4 antibody on the gold electrodes were employed. The immobilization method with protein A method gave the best results in terms of sensitivity, stability and reproducibility. The coated crystal with either polyethyleneimine adhesion, followed by a glutaraldehyde cross-linking method, or physical adsorption method, gave inferior results. A thin protein A membrane was formed on the gold electrodes so that C4 antibody could be effectively self-assembled, as an orderly, closely arranged and oriented antibody membrane on the protein A layer. Under the optimized experimental conditions, the coated crystal had good response to C4 in the range 5 x 10(-4) to 1 x 10(-2) g dm(-3). Other proteins in human serum did not interfere with the detection. The crystal could be regenerated nearly 10 times.