The Japanese pear ‘Osa-Nijisseiki’ (S 2 S 4 SM ) (SM = stylar-part mutant) is a self-compatible bud mutant that originated from self-incompatible ‘Nijisseiki’ (S 2 S 4 ). The S 4 -allele of the pear is deleted and it shows unilateral incompatibility to cultivar with an S 2 S 4 genotype. However, when pollen-tube growth was compared between cross-compatible [‘Osa-Nijisseiki’ × ‘Okusankichi’ (S 5 S 7 )], unilateral-compatible [‘Osa-Nijisseiki’ × ‘Kikusui’ (S 2 S 4 )], self-compatible (‘Osa-Nijisseiki’ × ‘Osa-Nijisseiki’), and unilateral-incompatible pollination (‘Kikusui’ × ‘Osa-Nijisseiki’), pollen-tube growth clearly showed the following order: cross-compatible > unilateral-compatible > self-compatible > unilateral-incompatible. This indicates that the ‘Osa-Nijisseiki’ style produces specific inhibitor(s) not only to S 2 - and S 4 -pollen but also “self-pollen”, because the phenotype of S 4 SM -pollen is the same as S 4 -pollen. Stylar protein analysis demonstrated that ‘Osa-Nijisseiki’ produces S 2 -RNase (RNase associated with S 2 -allele) together with a small amount of S 4 -RNase. The purified S 4 -RNase possessed almost the same inhibitory action on the growth of S 4 -pollen-tubes in vitro at 1μgμl −1 as that from original ‘Nijisseiki’. These results suggest that the depressed growth of unilateral-compatible and self-pollen-tubes in ‘Osa-Nijisseiki’ is due to this biologically active S 4 -RNase. Growth of self-pollen-tubes may also be depressed by inhibitor(s) specific to “self-pollen” unrelated to S-alleles.