Immature pig oocytes cultured for 36 h in a modified tissue culture medium 199B were freed from cumulus cells and treated for 30 min with fluorescein isothiocyanate-labelled lectins. When examined under fluorescence illumination, Ricinus communis agglutinin (RCA-I) and Lens culinaris agglutinin (LCA) bound to all oocytes, with the strongest fluorescence in either the outer region of the ZP (RCA-I) or throughout the ZP (LCA). However, Bandeiraea simplicifolia lectin-I (BS-I) and Ulex europaeus agglutinin-I (UEA-I) bound with either strong or weak intensity mainly to the outer and inner regions of the ZP, respectively, in～70-90% of oocytes examined. Bandeiraea simplicifolia lectin-II (BS-II) bound to various regions of the ZP with different intensities in only 60% of oocytes examined. At 2 h after insemination in vitro, significantly fewer spermatozoa were bound to the ZP of lectin-treated oocytes than untrearted control oocytes. Of the lectins, RCA-I and LCA most inhibited sperm binding. At 12 h after insemination, penetration rates were similar in control oocytes and those treated with BS-I or BS-II, but penetration rates were lower in oocytes treated with UEA-I than those in untreated controls, and an almost complete block of sperm penetration was observed in oocytes treated with RCA-I or LCA. The incidence of monospermy was similar in untreated oocytes and those treated with BS-I or UEA-I, but it was higher in oocytes treated with BS-II. These results suggest that β-D-galactose and α-D-mannose residues in the pig ZP may act as primary sperm receptors and/or inducers of the sperm acrosome reaction and β-D-N-acetylglucosamine residues may be involved in the block of polyspermy. The variability in binding of BS-II to the ZP, which correlated with monospermy/polyspermy may reflect differences in the maturation state of individual oocytes. In future, this lectin might provide a means of monitoring maturation in vitro, leading to the development of improved culture systems.