S-alleles of Brassica oleracea were identified using a method which is based on the amplification of S-sequences from genomic DNA, followed by digestion of the PCR products with selected restriction enzymes (PCR-RFLP). A study was made in which the same S-allele was present in the homozygous state in a range of different crop types. This showed that, with minor exceptions, characteristic restriction patterns were obtained, and therefore that it was possible to identify the S-allele. To test whether the method was also suitable for the identification of both the S-alleles present in heterozygotes, a number of S-heterozygotes together with an F2 population were screened. The results showed that the standard method was not very reliable for the identification of both of the S-alleles. This is because firstly, one of the S-alleles may be amplified preferentially, and secondly, the restriction patterns are not unique to a particular combination of S-alleles. Finally, although it is not possible to identify unequivocally both S-alleles of heterozygotes using a standard technique, the procedure can be modified for particular combinations of alleles to enable the identification to be made.