The mechanisms of self-compatibility in the Japanese plum cultivar, 'Methley' were investigated. Using PCR with S- RNase gene-specific primers, the S-genotype (S-haplotype combination) of 'Methley' was determined to be SbSg.This S-genotype was confirmed by cloning of the partial genomic DNA for both the S-RNase genes. After self-pollination of 'Methley',the resultant progenies segregated into three S-genotypes,SbSb,SbSg,SgSg,suggesting that both the Sb- and Sg-haplotypes were inherited in the progenies from the male parent. Thus, both S-haplotypes seemed to be non- functional and responsible for self-compatibility. Expression analyses in 'Methley' showed that both the Sb- and Sg- RNase genes were transcribed in the style.Therefore, it appeared that inhibition of transcription of the S-RNase genes was not responsible for self-compatibility in the Sb- and Sg-haplotypes of this cultivar. When flowers of the self- incompatible cultivars, 'Sordum' (SaSb) and 'White Plum' (SfSg) were fertilised with pollen from 'Methley', the progenies segregated into four S-genotypes, SaSb, SaSg, SbSb, SbSgand SbSf, SbSg, SfSg, SgSg, respectively. This result indicated that growth of the Sb- or Sg-pollen tubes of 'Methley' was not arrested in styles with functional Sb- or Sg- RNase alleles. Thus, the pollen components in both the Sb- and Sg-haplotypes of 'Methley' may be dysfunctional and involved in the breakdown of self-incompatibility. However, no alterations in the SFBbor SFBggene (S-haplotype-specific F-box gene) sequences of 'Methley' were detected, and both the SFBband SFBggenes were transcribed in pollen of this cultivar. Therefore, neither a mutation in the SFB gene sequences, nor any inhibition of transcription of the SFB genes appeared to be the cause of the loss function in the pollen component in Japanese plum.