Some properties of solar active regions at 9, 3.5 and 1.2 mm wavelengths are discussed. The regions have excess brightness temperatures of up to 1000, 700 and 150 K at 9, 3.5 and 1.2 mm wavelengths. The background radiation at 3.5 mm is often seen to be ‘absorbed’ in regions closely coincident with Hα dark filaments on the disk. Interpretation of this ‘absorption’ as due to the large optical thickness of the overlying filamentary material leads to an estimate of electron density in the filaments. The 9 and 3.5 mm-λ regions show almost one-to-one correspondence with the Ca-plage regions as well as with the regions on magnetograms. The latter relationship suggests the possibility of measuring chromospheric magnetic fields from the measurement of polarization at millimeter wavelengths.