The localisation of cyclin B throughout in vitro maturation of pig oocytes was determined by indirect immunofluorescence using a monoclonal antibody specific for an epitope of the human cyclin B. Maturation of pig oocytes was induced by addition of Pergonal (2 UI/ml of FSH/LH) and β-oestradiol to the medium where isolated ovarian follicles were cultured for up to 72 h. Immature gametes with an intact germinal vesicle were observed during the first 30 h of culture. Only 10% were competent to reinitiate meiosis and showed germinal vesicle breakdown (GVBD) after 36 h. However, after 48–72 h, 60% of the oocytes accomplished their maturation and showed metaphase chromosomes. Immature oocytes showed cyclin B immunofluorescent staining in the cytoplasm, whereas mature oocytes showed the immunofluorescent label concentrated in the nucleus. Metaphase chromosomes showed an intense immunofluorescence. The migration of cyclin B to the nucleus and its association with metaphase chromosomes in pig oocytes able to progress through meiosis resembled the subcellular localisation of cyclin B and the distribution of maturation promoting factor (MPF) in mitotic dividing cells.