Thermal expansion, temperature change of electrical resistance and its change due to hydrostatic pressure, and magnetic susceptibility were measured with some antiferromagnetic Fe-Mn alloys containing 30 to 40 at% Mn. It was found that an additional magnetic volume expansivity, δV/V, due to an antiferromagnetic spin ordering is as large as 10^?10^, and the change of the Neel temperature with pressure, ∂T_N/∂P is about -2.5×10^ deg. Kg^・cm^2 in 30 at% Mn alloy. With these values, the volume dependence of the molecular field constant, ∂A/∂ω) is estimated to be positive by the molecular field theory with the localized moment model. The results are discussed in comparison with Bethe-Slater\u27s and Weiss\u27 theories and with the similar behavior in some ferromagnetic invar-type alloys
