We report an investigation of the antiferromagnetic Heusler metal Mn₃Si and semi-Heusler metal CuMnSb. The Néel-temperatures T_N \approx 23K for Mn₃Si and T_N \approx 50K for CuMnSb as seen by a pronounced anomaly in the specific heat are not affected by a magnetic field up to 14 T. The magnetisation is unsaturated, but does not show any signs of metamagnetic transitions in the ordered state. The resistivity drops at the onset of antiferromagnetism, but is not affected by magnetic field apart from the residual value for T \to 0, which increases slightly. This invariance of the antiferromagnetic order to high magnetic field is incompatible with present day models for magnetism in metals, and suggests an unconventional form of antiferromagnetic order.

PACS numbers: 75.40.Cx, 71.45.Lr, 75.30.Fv, 75.50.Ee