An immunosensing system is described for the detection of specific antibodies in human sera using a piezoelectric immunosensor and a flow through system. A quartz crystal microbalance theory, regarding mass and viscoelastic effects in multilayer systems, which is derived from the Mason circuit, gives the theoretical background for the interpretation of measurement data obtained from an oscillator circuit. An acoustic model for the antigen/antibody reaction is described (Part 1). An immunospecific detector was developed by immobilizing synthetic peptides or recombinant proteins, comprising major diagnostic epitops of HIV, on the surface of the transducer. Attempts have been made to use the sensor for the initial screening for antibodies, specific against the Human Immunodeficiency Virus (HIV), in human serum samples. Results are obtained within a few minutes and with a selectivity comparable to a licensed HIV ELISA (Part II).