Some properties of solar active regions at 9 and 3.5 mm wavelengths are discussed. The regions have excess brightness temperatures of up to 1000 and 700K at 9 and 3.5 mm wavelengths respectively. 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 active regions at millimeter wavelengths show almost one-to-one correspondence with the Ca-plage regions as well as with the regions of longitudinal magnetic fields on Mt. Wilson magnetograms. A comparison of the mm-λ maps with the magnetograms ‘smoothed’ with the beams of mm observations shows this correspondence in a striking manner. This relationship suggests the possibility of measuring chromospheric magnetic fields from the measurement of polarization at millimeter wavelengths.