A piezoelectric immunosensor has been developed for the determination of human IgM. The crystals are AT-cut and have a basic resonant frequency of 9 MHz. Immobilization of goat antihuman IgM antibodies to the crystals' surfaces was accomplished via a CNBr-activated copolymer coating of 2-hydroxyethyl methacrylate and methylmethacrylate. The IgM piezoelectric immunosensor can be used for the human IgM determination in the range 5-93 micrograms ml-1. The analytical results given by this approach were in satisfactory agreement with those given by the single radical immunodiffusion procedure. The sensitivity, specificity and reproducibility of this immunosensor were investigated. Further, the valent value of goat antihuman IgM antibody binding with human IgM antigen and the affinity constant of immunoreaction in this experimental system were studied. After washing with tetrahydrofuran, a crystal can be re-used 20 times without detectable loss of sensitivity.