A new group of putative membrane receptors have now been isolated from fish and other vertebrates, including human. These proteins are classified into three groups known as membrane progestin receptor alpha, beta and gamma (mPR-α, -β and -γ). In the present study we have investigated the role of mPR-β in regulating in vitro maturation (IVM) of pig cumulus–oocyte complexes (COCs). RT-PCR and Western blot analysis indicated that COCs contain transcripts and proteins for mPR-β. The levels of both transcripts and proteins increased between 0 and 20 h IVM, but then decreased between 20 and 44 h. The luteinizing hormone (LH) and follicle-stimulating hormone (FSH) did not affect mPR-β expression during IVM. Immunofluorescence analysis indicated that the mPR-β was localized in the plasma membrane of cumulus cell. However, in mouse embryonic fibroblasts (MEFs), mPR-β was detected at the endoplasmic reticulum (ER) rather than the plasma membrane. Cumulus expansion was impaired significantly (P < 0.05) when COCs were incubated in maturation medium containing 10% (v/v) anti-mPR-β serum during IVM. Bioinformatics analysis predicted that mPR-β had an ER retention motif and an endocytosis internalization motif. These results suggest that the mPR-β is a molecule related to cumulus expansion and it might function by regulation of exocytosis. In conclusion, this is the first description of the expression patterns and subcellular localization of mPR-β in COCs and might shed light on the function of the protein.