Abstract. The weighted mean matrix   ${{M}_{a}}$  is the triangular matrix
 ${{M}_{a}}$  is the triangular matrix   $\left\{ {{a}_{k}}/{{A}_{n}} \right\}$ , where
 $\left\{ {{a}_{k}}/{{A}_{n}} \right\}$ , where   ${{a}_{n}}\,>\,0$  and
 ${{a}_{n}}\,>\,0$  and   ${{A}_{n}}\,:=\,{{a}_{1}}\,+\,{{a}_{2}}\,+\cdots +\,{{a}_{n}}$ . It is proved that, subject to
 ${{A}_{n}}\,:=\,{{a}_{1}}\,+\,{{a}_{2}}\,+\cdots +\,{{a}_{n}}$ . It is proved that, subject to   ${{n}^{c}}{{a}_{n}}$  being eventually monotonic for each constant
 ${{n}^{c}}{{a}_{n}}$  being eventually monotonic for each constant   $c$  and to the existence of
 $c$  and to the existence of   $\alpha \,:=\,\lim \,\frac{{{A}_{n}}}{n{{a}_{n}}},\,{{M}_{a}}\,\in \,B\left( {{l}_{p}} \right)$  for
 $\alpha \,:=\,\lim \,\frac{{{A}_{n}}}{n{{a}_{n}}},\,{{M}_{a}}\,\in \,B\left( {{l}_{p}} \right)$  for   $1\,<\,p\,<\infty $  if and only if
 $1\,<\,p\,<\infty $  if and only if   $\alpha \,<\,p$ .
 $\alpha \,<\,p$ .