The criteria for lasing in a dielectric sphere surrounded by an active medium are given. They are presented in simple analytical form and confirmed by numeric calculations of the amplitudes of the morphology-dependent resonances in spontaneous emission spectra. The determined lasing threshold of this novel-type laser is compared with that of a conventional spherical laser with an optically active internal layer. The diverse advantages of a laser based on the gain of the evanescent part of a whispering-gallery mode are discussed.