We describe the first example of a predominantly nuclear protein which during mitosis becomes part of the mitotic apparatus. This protein has been termed the nuclear-mitotic apparatus (NuMA) protein. In interphase cells NuMA protein is restricted to the nucleus and is a constituent of isolated nuclear matrices, but in mitotic cells it is observed by indirect immunofluorescence microscopy to be concentrated at the polar regions of the mitotic apparatus. This mitotic localization is dependent on the integrity of the spindle, since treatments which disrupt the spindle result in dispersion of NuMA protein throughout the cell. Comparison to the subcellar distribution of tubulin at different stages of the cell cycle indicates that NuMA protein is distinct from the previously identified components of the mitotic spindle. Its association with the nuclear matrix and its localization during mitosis to the site of nuclear reassembly suggest the interesting possibility that NuMA protein could be representative of a class of proteins involved in the early events of nuclear reassembly. NuMA is present in the nuclei and mitotic spindle of all types of human cells that have been examined, but proteins of similar molecular weight (300,000 daltons in dissociating solvents) or immunological specificity are not detected in cells of other species (including monkey). However, the NuMA protein is synthesized in a human/Chinese hamster hybrid cell containing a reduced number of human chromosomes. Immunofluorescence studies of this hybrid cell showed that the distribution of NuMA protein is equivalent to that in human cells. These results suggest that the human gene coding for NuMA protein, unlike other genes coding for human specific nuclear proteins, can be expressed in human/hamster hybrid cells and that the cell hybrids will be useful in further characterization of NuMA protein.