Abstract: A new bulk acoustic wave (BAW) sensor modified with a molecularly imprinted polymer (MIP) was fabricated and applied for the: determination of pyrimethamine. This sensor exhibited high selectivity and sensitive response to pyrimethamine. Factors such as pH and the amount of coating influencing sensor properties, were investigated in detail and optimized. The calibration curve was linear in the range 6.0 x 10(-7)-1.0 x 10(-4) M. The determination limit was 2.0 x 10(-7) M. In harsh chemical environments such as high temperature, organic solvents, bases, acids, etc., the sensor still exhibited long- term stability. The proposed method has been satisfactorily applied for the determination of pyrimethamine in serum and urine media. (C) 2000 Elsevier Science B.V. All rights reserved