  Within the QCD sum-rule approach in an external field, we calculate the
baryon matrix element of isovector-scalar current, $H_{\rm B}=\langle
B|\overline{u}u- \overline{d}d|B\rangle/2M_{\rm B}$, for octet baryons, which
appears in the response of the correlator of baryon interpolating fields to a
constant isovector-scalar external field. The sum rules are obtained for a
general baryon interpolating field with an appropriate form for the
phenomenological ansatz of the spectral density.
  The key phenomenological input is the response of the quark condensates to
the external field. To first order in the quark mass difference $\delta
m=m_d-m_u$, the non-electromagnetic part of the baryon isospin mass splitting
is given by the product of $\delta m$ and $H_{\rm B}$. Therefore, QCD sum-rule
calculation of $H_{\rm B}$ leads to an estimate of the octet baryon isospin
mass splittings. The resulting values are comparable to the experimental
values; however, the sum-rule predictions for $H_{\rm B}$ are sensitive to the
values of the response of the quark condensates to the external source, which
are not well determined.
