In this study the morphological and protein diversity of twelve diploid perennial ryegrass accessions (Lolium perenne L.) was examined. These accessions comprised five closely related groups, each containing an `initial variety' (IV) and one or more declared `essentially derived varieties' (EDV), with differing degrees of relatedness. `Essential derivation' is a legal concept relating to intellectual property in plant varieties and is additional to Plant Breeders Rights (PBR). An EDV is defined as clearly distinct from, but conforming in its expression of the essential characteristics of an IV, from which it is found to have been predominantly derived. Where an essential derivation has been confirmed, the breeder of the IV may be entitled to some royalty sharing of the EDV. Clearly, therefore, in any successful EDV claim, evidence of a high degree of conformity in either the phenotype or genotype would be required. Examination of plant morphology indicated that all the EDVs were morphologically distinct from their corresponding IV in one or more morphological characteristic. using a principal co-ordinates analysis to give an overall measure of morphological difference, all twelve accessions were correctly clustered into their related groups and the magnitude of the differences within groups reflected their known breeding histories. Examining protein diversity by methods that targeted single- and multiple-locus genes also clustered the accessions into their correct groups, in most cases. However, only by examining diversity in seed storage proteins were within-group relationships accurately represented. The methods used provided no consistent representation of between-group relationships. It was concluded that the morphological method provided a creditable measure of genetic conformity, but to avoid incurring excessive time, work and cost, results from existing national PBR trials would need to be openly available. Within the limits of the genetic material examined, seed storage protein diversity appeared to provide a suitable combination of accuracy and efficiency on which to base a routine test. However, given more complex breeding relationships than those in this study, methods such as AFLP(1) markers that sample more genetic diversity, may be necessary to maintain this level of accuracy.