We have used quantum-cascadlea sers operating at 4.7THz, 3.5THz, and 2.3THz to perform THz magnetoopticals tudies of semiconductors. We observedc yclotronr esonance in In As and In Sb quantumw ellsf rom liquid-heliumt emperaturesto room temperature. This representst he first spectroscopica pplicationof THz quantum-cascadela sers. Results showt hat these compact lasers are convenient and reliable sources with adequatep ower and stability for this type of far-infraredm agneto-opticaslt udyo fsolids. Their compactness promisesi nterestingf uture applicationsi nsolid-states pectroscopy. As an example, the developmento fan ultracompacot ptically-detected THz resonances pectroscopys ystemwill be described.