In system identification it is important that any a priori information about the system is utilized in the processing of measured data. Structural vibration problems can be modelled in terms of mass, stiffness, and damping and it is usually the case that the mass matrix is positive definite. In this paper the authors demonstrate how the property of positive definiteness of the mass matrix can be included in the formulation of a continuous frequency domain filter. Results are compared with those obtained from a linear, frequency domain filter which does not impose the condition of positive definiteness on the mass matrix.