The localization of PCNA was demonstrated in paraffin sections of pig ovarian tissue using primary mouse monoclonal anti-PCNA antibody. In primordial follicles, no remarkable staining for PCNA either in granulosa cells or in the oocytes was observed. Inprimary to secondary follicles, positive staining in oocytes and in some of granulosa cells was detected. The advanced preantral and particularly actively growing small to large antral follicles showed extensive PCNA labelling in the layers of granulosaand theca cells and in the cumulus cells encircling the oocyte. PCNA labelling was expressed in nuclei of oocytes in preantral and small antral follicles.