Localization sites and labeling intensity of alphaB-crystallin in corpus luteum (CL) of pseudopregnant rats has been studied using postembedding light and electron microscopical immunohistochemistry. At days 2 and 18 alphaB-crystallin labeling was found to be significantly higher compared with the luteal maintenance period (days 6 and 10). alphaB-crystallin localized both to luteal and interstitial cells of CL. At light microscopical level alphaB-crystallin labeling decreased during CL life span in the central area of luteal cells cytoplasm and increased in the peri-plasmalemmal area. Interstitial cells labeling was found to increase at day 6, followed by almost complete disappearance during functional luteolysis (days 15 and 18). Our results at electron microscopical level showed alphaB-crystallin to localize in cytoplasm with close relationship with endoplasmic reticulum, Golgi apparatus, mitochondria and also in nuclei of luteal and interstitial cells. At day 2 labeling of luteal cells was abundant in cytoplasm but weak in nuclei. During the luteal maintenance period and functional luteolysis labeling in luteal cells was relocalized to the peri-plasmalemmal and perinuclear areas. Labeling in nuclei of luteal cells was weak. At the same time (day 6) interstitial cells including nuclei showed strong labeling, which was significantly decreased during luteolysis. Immunohistochemically detectable tubulin decreased in CL tissue during CL life span allowing to suggest that alphaB-crystallin acts as chaperone, one possible role of which is to stabilize the cytoskeleton in different CL cell types during CL formation and active functioning.