  The mixture of bond-alternating and uniform S=1/2 antiferromagnetic
Heisenberg chains is investigated by the density matrix renormalization group
method. The ground state magnetization curve is calculated and the exchange
parameters are determined by fitting to the experimentally measured
magnetization curve of \CuCl$_{2x}$Br$_{2(1-x)}$($\gamma$-pic)$_2$. The low
field behavior of the magnetization curve and low temperature behavior of the
magnetic susceptibility are found to be sensitive to whether the
bond-alternation pattern (parity) is fixed all over the sample or randomly
distributed. The both quantities are compatible with the numerical results for
the random parity model.
