Low-temperature magnetic isotherms of &agr;-Mn and its alloys with Fe, N, and H have been examined.Hysteresis loops and effects of cooling in an applied field are reported. Similar properties are found forsingle crystals of Cu-Mn. It is pointed out that the spin density wave mechanism of antiferromagnetism,proposed by Overhauser, provides explanations for these phenomena. Neutron diffraction data are reinterpretedand provide further evidence for spin density waves in &agr;-Mn and its alloys.