  We propose a scaling law for the onset of turbulence in pipe flow of
neutrally buoyant suspensions. This scaling law, based on a large set of
experimental data, relates the amplitude of the particle-induced perturbations
($\epsilon$) to the critical suspension Reynolds number, $Re_{s,c}$. Here
$\epsilon$ is a function of the particle-to-pipe diameter ratio and the volume
fraction of the suspended particles, $\epsilon = (d/D)^{1/2} \phi^{1/6}$.
$Re_{s,c}$ is found to scale as $\epsilon^{-1}$. Furthermore, the perturbation
amplitude allows a distinction between classical, intermediate and
particle-induced transition.
