In this paper, we take a fresh look at the determination of the
transition matrix for a homogeneous, biisotropic particle employing the
Null-field approach. The condition for passivity of the material is
discussed. In particular, we focus on some previously unproved
properties of the Beltrami spherical vector waves, such as completeness,
orthogonality, and linear independence, which are all instrumental in
the analysis of the scattering problem with the Null-field approach.
Some numerical examples illustrate the approach.