Determination of Staphylococcus aureus was performed by using a piezoelectric crystal immunosensor system. The system consists of an oscillator, a frequency counter, a special cell and a modified piezoelectric crystal. Goat-anti-human IgG is immobilized on the surface of the crystal, which is used as a S. aureus detector by measuring its resonant frequency shift due to a mass change caused by specific binding of the microbe to the crystal surface. The frequency shift was correlated with a S. aureus concentration in the range of 5 x 10(5) -1 x 10(8) cells. The coated crystal was stable for fifteen assays without detectable loss of sensitivity and stable for a month when it was stored at + 4 degrees C.