  Magnetization process of ferrimagnetic Heisenberg chains of alternating spins
are theoretically studied. The size scaling analysis with the exact
diagonalization of finite systems for ($S$,$s$)=(3/2,1) and (2,1) indicates a
multi-plateau structure in the ground-state magnetization curve for $S$ and $s$
$>1/2$. The first plateau at the spontaneous magnetization can be explained by
a classical origin, that is the Ising gap. In contrast, the second or higher
one must be originated to the quantization of the magnetization. It is also
found that all the $2s$ plateaux, including the classical and quantum ones,
appear even in the isotropic case with no bond alternation.
