A piezoelectric immunoassay technique, which is based on the detection of agglutination of antibody- or antigen-bearing polymer by an immunoreaction using a piezoelectric quartz crystal, has been developed for the determination of complement III (C3). Anti-C3 antibodies were physically adsorbed onto the carboxymethyl cellulose polymer by hydrogen bonds. The agglutination of the (anti-C3 antibody)-bearing polymer by immunoreaction caused a viscosity change of the solution, which could be monitored by a piezoelectric quartz crystal. The effect of experimental conditions such as the polymer concentration, the antibody dilution ratio and the reaction temperature on the frequency response were investigated. The linear ranges for C3 concentration determined by the end-point method and the initial rate method were 22.0-43.2 micrograms ml-1 and 22.0-49.1 micrograms ml-1, respectively. Other antigens presenting in serum did not interfere with the determination of C3. Analytical results of ten clinical specimens obtained using the developed technique were in satisfactory agreement with those given by the rate diffusion turbidimetry. With a simple regeneration method devised, the crystal can be used repeatedly with acceptable reproducibility.