The formation and maintenance of the bipolar mitotic spindle apparatus require a complex and balanced interplay of several mechanisms, including the stabilization and separation of polar microtubules and the action of various microtubule motors. Nonmicrotubule elements are also present throughout the spindle apparatus and have been proposed to provide a structural support for the spindle. The Nuclear-Mitotic Apparatus protein (NuMA) is an abundant 240 kD protein that is present in the nucleus of interphase cells and concentrates in the polar regions of the spindle apparatus during mitosis. Sequence analysis indicates that NuMA possesses an unusually long alpha-helical central region characteristic of many filament forming proteins. In this report we demonstrate that microinjection of anti-NuMA antibodies into interphase and prophase cells results in a failure to form a mitotic spindle apparatus. Furthermore, injection of metaphase cells results in the collapse of the spindle apparatus into a monopolar microtubule array. These results identify for the first time a nontubulin component important for both the establishment and stabilization of the mitotic spindle apparatus in multicellular organisms. We suggest that nonmicrotubule structural components may be important for these processes.