As recently as 1968 the only molecules known to exist in interstellar space were CH, CH+ and CN, identified by means of their absorption spectra superimposed on the optical spectra of bright stars, and OH which in its ground state emits at a radio wavelength of 18 cm. Since then there has been a remarkable series of discoveries of about twenty additional molecules emitting in the radio spectrum, some of them quite complex (CH3OH for example). But of the originally discovered `optical' radicals, only CN, emitting at about 2.6 mm, has been detected at radio frequencies-in the Orion Nebula, in the HII region W51, and in the infrared object IRC+10216. The other radical, CH, has not been found, mainly because the radio frequencies of its ground state lambda doublet have not been found in the laboratory, and it has not been possible to compute them theoretically with sufficient accuracy. The first estimate of the (main line) frequency seems to have been made by Shklovsky1: 3,180+/-10 MHz.