Most apple varieties have gametophytic self-incompatibility and are not capable of self-fertilization. Cultivars with one common S allele have reduced compatibility and do not reach their yield potential when planted nearby. Full variety compatibility occurs when both S alleles are different in parents. When crossing apple genotypes with full compatibility, all pollen grains of one plant are able to pollinate another plant. Information on the cross-compatibility of varieties is essential when selecting parental forms in selection programs and for matching pollinators in commercial gardens. As for apple varieties of foreign selection from S-locus alleles encoding another S-RNase, the most common allele is S3 (16.9%), followed by S7 (12.3%), S1 and S2 (10.8% each), S5 (8.7%), S9 (5.6%), S10 (5.4%), S24 and S28 (4.9% each), S20 (3.1%), S25 (2.6% ). In Russia, genotyping of apple varieties is carried out for a limited number of S-locus alleles. The most widespread allele of S locus among the genotypes of apple trees of the Russian selection with a fully established allelic composition is S10 (26.8%), followed by S3 (23.2%), S7 (16.1%), S2 (14.3% ), S5 and S9 (8.9% each). To identify full allelic S-locus status of apple varieties of domestic selection, to establish cross-compatibility and the best pollinators, it is necessary to carry out additional PCR analysis for other S-alleles, especially for the following alleles S1, S20, S22, S23, S24, S25 and S28. The article assesses 195 foreign apple genotypes and 58 Russian genotypes according to their S-allelic composition in order to provide selectors and apple producers with information on incompatibility among varieties. Characteristics of DNA markers, the nucleotide sequence of the primers, the sizes of the target fragments of the PCR product for detection