A CMOS MEMS platform for gravimetric biosensing in liquid has been fabricated and tested. The concept is an aluminum and silicon dioxide membrane, coated with polystyrene and functionalized with receptors. When the protein analyte binds to the receptors, the sensor mass increases which causes a shift in the resonant frequency. The device is electrostatically actuated and the displacement is sensed with piezoresis- tors on the membrane edges. The MEMS membrane is expected to have a better mass sensitivity than previous millimeter scale devices (SAW, QCM, FPW), because it has a higher surface area to mass ratio. In addition, it can be actuated to oscillate at higher resonance modes and this may improve the quality factor in liquid. Initial experiments with a scanning laser Doppler vibrometer have resulted in the frequency response and modes of the membrane. This demonstrates the ability to actuate the membrane in higher modes and the corresponding resonant frequency values that should be detected through the piezoresistors. Future work includes verifying the optical results with elec- trical tests of the piezoresistors.