An ultralow resonant frequency vertical seismic activity attenuation filter prototype has been designed and built as the vertical component of a Pre-Isolation Stage (PIS) for low-frequency Seismic Attenuation (SA) chains. The task of the PIS is to reduce the excitation of the chain payload's resonant residual motions. A characterization and the performance of the prototype are reported along with some design considerations. An attenuation of 45 dB has been achieved in the region of interest (the SA chain resonant frequency band) and an upgrade to more than 50 dB attenuation is easily achievable, Tests of active feedback loop attenuation driven by electro magnetic actuators and controlled by a Digital Signal Processor reading a precision vertical accelerometer have been successfully performed. The filter also effectively acts as a precision positioner for the suspended payload, (C) 1999 Elsevier Science B.V. All rights reserved.