Within the density-functional theory, including its local spin-density approximation, the fully relativistic calculations of electron, optical and magneto-optical spectra in some half-metallic ferromagnets and uranium monochalcogenide have been performed. An increase in the magneto-optical Kerr effect in half-metallic magnets is closely related to such factors as the spin-orbit exchange splittings and the plasma frequency. At the same time in the usual ferromagnets the peculiarities of band structure lead to anomalies in the conductivity tensor at energies close to the spin-orbit splitting.