  Let $\mathfrak{M}$ be a von Neumann algebra and let $A$ be a nonzero positive
element of $\mathfrak{M}$. By $\sigma_A(T) $ and $r_A(T)$ we denote the
$A$-spectrum and the $A$-spectral radius of $T\in\mathfrak{M}^A$, respectively.
In this paper, we show that $\sigma(PTP, P\mathfrak{M} P)\subseteq
\sigma_A(T)$. Sufficient conditions for the equality $\sigma_A(T)=\sigma(PTP,
P\mathfrak{M} P)$ to be true are presented. Also, we show that $\sigma_A(T)$ is
finite for any $T\in\mathfrak{M}^A$ if and only if $A$ is in the socle of
$\mathfrak{M}$. Next , we consider the relationship between elements $S$ and
$T\in\mathfrak{M}^A$ that satisfy one of the following two conditions: (1)
$\sigma_A(SX)=\sigma_A(TX)$ for all $X\in\mathfrak{M}^A$, (2) $r_A(SX)=
r_A(TX)$ for all $X\in\mathfrak{M}^A$. Finally, a Gleason-Kahane-\.Zelazko's
theorem for the $A$-spectrum is derived.% Finally, we introduce and study the
notion of the $A$-approximate point spectrum for element of
$X\in\mathfrak{M}^A$.

Operator Algebras;Functional Analysis