  A constraint on a long-range spin-dependent interaction
$\alpha^{\prime\prime}({\bf s}_1\cdot{\bf s}_2)\,{\rm e}^{-\lambda r}/r$, which
can be induced by a pseudovector light boson, is presented. We study
theoretical and experimental data on a specific difference $8\times E_{\rm
hfs}(2s)-E_{\rm hfs}(1s)$ for light two-body atoms. The spin-dependent coupling
constant $\alpha^{\prime\prime}$ of electron-nucleus interaction in hydrogen,
deuterium and helium-3 ion is constrained at the level below a part in
$10^{16}$. The derived constraints are related to the range of masses below
$4\;{\rm keV}/c^2$.
