To study the distribution of a major Ca(2+)-sequestering site in PtK2 cells, a rat kangaroo kidney epithelial cell line, during interphase and mitosis, we prepared an affinity-purified polyclonal antibody against bovine liver calreticulin (CRT), a major Ca(2+)-binding protein of the endoplasmic reticulum (ER). Immunofluorescence microscopy and immunoperoxidase electron microscopy showed that the anti-CRT antibody labeled a continuous reticular network of the ER and the nuclear envelope in interphase PtK2 cells. The same PtK2 cells double-stained with DiOC6 (3) and the anti-CRT antibody revealed labeling of identical reticular membranes. In contrast to the localization in the ER localization, the mitochondria and the Golgi apparatus were not labeled. These results confirm the exclusive localization of CRT in the ER and that this organelle is a major site for Ca2+ storage in non-muscle cells. In mitotic cells, marked changes of the labeled structure began at prophase-prometaphase and persisted throughout all phases of mitosis. The cytoplasm of the mitotic cells showed diffuse fluorescence, this being more intense around, but not inside, the mitotic spindle. Confocal microscopy and immunoelectron microscopy demonstrated that the CRT-containing membranes changed to segmented tubuloreticular structures, which were concentrated around the mitotic spindle. The ER containing CRT could be responsible for the sequestration of Ca2+ and for the regulation of the concentration of this cation during mitosis, as well as during interphase.